I'm inclined to think, yes, that if a law is unjust it should lose its status as a law, but it doesn't immediately follow that one has no obligation to follow that law. In the case of profoundly unjust laws, such as those regarding Jews in Nazi Germany, presumably it would, at least, be permissible not to follow the law. But there are many other laws that might be regarded as unjust that are far less oppressive. One might suppose one had an obligation to follow such laws out of a kind of respect for the law. What one ought to do instead might be to attempt to get the law changed. Of course, in some cases, publicly and openly refusing to follow the law might be a way of drawing attention to its injustice: That's civil disobedience, in its simplest form.

It is worth distinguishing two issues: anger that another country applies its laws to one of our citizens; and anger that another country (any country, actually) applies unjust laws to one of our citizens (to anyone, actually).

Though your question seems more focused on the former issue, I think you are really more exercised by the latter. You would be very strongly opposed to Australia hanging a "mule" and to Australia punishing a man for having homosexual relations. And you would not be upset, I guess, if an Australian abroad received a punishment you consider just for some culturally specific crime that does not exist in Australia (e.g., having sex with a woman on the basis of a false promise to marry her while knowing that such sex will make her a total and permanent outcast in her community).

As it happens, I am also in Australia and saw a poll last night showing that 47% of Australians want the hanging to go ahead, while 46% are opposed to it. I suppose those 47% think it is alright for someone to be hanged for smuggling 400 grams of heroin and, given that they find this alright, they don't mind it being done by foreigners (assuming the man had a fair trial and so on).

In sum, then, I think we should think about criminal law at home and abroad together, against the background of one single conception of justice. This conception must recognize that the criminal law must be adapted to cultural conditions which may vary from country to country. (In Australia, justice would not be served by jailing men for using a false promise of marrige to entice a women to have sex; but in some very religious countries justice might well be served by such a law.) This conception must further recognize that in some countries harsher punishments for some offense may be appropriate because this type of offense is there more frequent or more harmful or harder to suppress (think of the offense of lighting a fire without a permit). And this conception must also recognize that there is a considerable element of judgment involved in settling upon appropriate punishments: There can be reasonable disagreement over whether a drug smuggling offence of the sort you cite should, in a context like Singapore's, fetch six months in jail or two years. But, contrary to what 47% of your compatriots seem to believe, hanging this young man is a clearcut injustice.

Philosophers of law spend a lot of time arguing over this question. This is not surprising since, on the one hand, there seems clearly to be some close connection; but on the other hand there are both actions that are immoral but not illegal (e.g. not keeping a promise) and actions actions that are illegal but not immoral (e.g. breaking an law requiring people to turn in escaped slaves).

Some so-called natural lawyers have claimed that the idea of an immoral law is an oxymoron. If some state diktat says that people of a certain race can't travel into certain areas, then that's not a law. That's fine -- but essentially it involves giving a new and special meaning to the word 'law'. Law and morality can be seen as analogous in various ways. They have a similar structure (both involve requirements, permissions, demands, etc.); they serve similar functions (such as coercing people into certain behaviour for social purposes); and they probably have similar origins (see e.g. the work of the anthropologist Christopher Boehm on this). If one sees both law and morality as essentially forms of social coercion, then one is not a branch of the other. In the case of each, we can ask ourselves whether we have a reason to accept it, or parts of it, and whether it can be improved in some way or other.

It's worth distinguishing between what one is free to do and what value to one that freedom has. Perhaps you're right that in a world in which there was no political society (a State of Nature, as some political philosophers call it) we would be free to do many more things than we are now (since no laws would exist that restrict our freedom). But the worth of those freedoms would be very small. Yes, we'd be free to travel wherever we wanted (without the need for passports, etc.), but most likely, absent the security that a political society provides, the level of industrial development would be so low that there would be no cars, no planes, no roads, etc. Even if there were roads, it would be so very dangerous to set out on them that I wouldn't dare risk it. Whereas now, my freedom to travel is worth something to me: I can drive (I have a car, I can buy fuel for it, there are roads!) confidently to the airport (there are airports!) and take a plane (there's an aerospace industry!) to Reykjavik. The freedom to fly to Reykjavik isn't worth much if there are no planes, no cars, no roads, no safe traveling.

So, even if you thought that freedom (the absence of conduct-regulating laws enforced by the power of the state) is a good thing and that freedom would be increased living in a State of Nature, reasonable people might still choose to live in a political society with a government that restricts their freedoms, because the freedoms they would have would be of value to them. (See also Question 291.)