According to a recent survey of philosophers, a majority —but not a large majority—would tend to agree that there are objective moral truths. But the minority who don't is not small. So yes: there are "many" philosophers who don't believe in objective moral truths.
Now these philosophers would say it's not true that it's always wrong to torture people purely for pleasure. Of course, this doesn't mean that they think it's okay to torture. They think that moral claims aren't the sorts of things that can be true. But why?
The easiest way to get a feel for this is by appeal to the old chestnut that "is" doesn't imply "ought." No statement of non-moral facts ever entails a moral claim. We might be revolted by what torture amounts to, but "torturing people for pleasure revolts me" doesn't add up to "torturing people for pleasure is wrong"; there's a logical gap between "X revolts me" and "X is wrong."
This isn't enough by itself. After all, there's a gap between biological truths and mathematical truths. But that doesn't mean there are no mathematical truths. Maybe there's a separate category of moral facts, and the wrongness of torturing for fun is among them. But the second arrow in the anti-realist's quiver is aimed at the claim that moral facts are somehow supposed to put objective constraints on our behavior. It's one thing to say that if we want to accomplish some optional goal, we need to do what getting to the goal requires. It's another thing to say that some special kind of truth can oblige us to act in a certain way even if we don't recognize the obligation and regardless of whether there's anyone to enforce it. The difficulty is part of what the philosopher J. L. Mackie called the "queerness of morality". If there are moral facts, they are apparently quite different from all other kinds of facts, or so the story would go.
So it's not that these philosophers think it's okay to be gratuitously cruel. It's a safe bet that most of them are just as distressed by torture as you are. Their point is at a much more abstract level: they don't think there are moral facts, because they don't see how there could be. But that's consistent with believing that overall, life will go much more smoothly if we act as well-known moral principles would require, with believing that this will help prevent pain and produce more joy, with wanting whole-heartedly for fewer people to suffer and more to live happy lives, and with doing what they can to bring all that about.