I don't read philosophy, so I can't tell if someone has said it exactly like this, but it seems to me that this is not a strawman. At least it seems to me that I have heard such ideas floating around, although not expressed this clearly. (Maybe it's not exactly what the original philosopher said; maybe it's just a popular simplification.) There is the unspoken assumption that when people "do what they want", that does not include caring about others; that people must be forced into pro-social behavior... and the person who says this usually suggests that some group they identify with should be given power over the evil humans to force them into doing good.

I do read philosophy, and this does seem like a strawman to me. I'm not aware of a single serious moral philosopher who believes there is a sharp dichotomy between "doing what you want" and "helping others".

The only philosopher who comes close, I think, is Kant, who thought that the reasons for performing an action are morally relevant, above and beyond the action and its consequences. So, according to Kant, it is morally superior to perform an act because it is the right thing to do rather than because it is an act I want to perform for some other reason. Given this view, the ideal test case for moral character is whether a person is willing to perform an act that goes against her non-moral interests simply because it is the right thing to do. But this still differs from the claim that altruistic behavior is opposed to self-interested behavior.