If we take your description ("usurping the copyright") at face value, then the answer is no, it isn't ethical. It's usurping, which is defined as seizing and holding something without legal right to do so; barring some highly unusual circumstances (and certain views about the relation of law to morality), that's going to be unethical. Similarly, "copyright" is by definition a legal protection.

If "usurping the copyright" isn't precisely what you meant, but rather are curious about the ethical status of using internet content without the permission of the original source, then you've hit on an important and fascinating issue, from both the ethical and the legal standpoints. It's an issue still in the process of being worked out. Some internet content is explicitly copyrighted -- sometimes technically so, sometimes by virtue of the site owner slapping a copyright symbol on the page. Not beign a copyright lawyer, I'm not sure what difference this would make to a judge. But it does indicate that the source intends for it to be used only with permission, or at least attribution, and all else equal, I think it would be unethical to use it without an attempt to obtain permission, and without attribution.

Without the explicit copyright claim, I would still err on the side of trying to obtain permission from either the author or (since authors are often anonymous or pseudonymous) the site owner. Many sites have links to instructions, "How to cite this article." To deliberately disregard this would, to my mind, be unethical. Certainly to present someone else's content as your own, explicitly or even implicitly, would be unethical, as it amounts to deception.

In the case of AskPhilosophers.org, clicking "About this site" shows you this: "Content on this site may be reproduced without the site owner's permission only if it is used for educational and not-for-profit purposes. For all other purposes, the site owner's permission is required (at the above e-mail address). In all cases, AskPhilosophers.org must be acknowledged (as well as any panelists whose responses are used). For further information, see also here." So if one of us panelists posts the meaning of life in answer to an anonymous question, we'd have to be given credit and the anonymous questioner acknowledged.