They can, but it would be rude if the developer has asked you not to, the price is low enough that everyone can afford it, and you didn't contribute anything of value.

Indeed, not just rude, but you might find if you keep doing that, that a promising project is abandoned, and nobody else takes it up. If that happens, you haven't done the community any favours. Of course it's hard to know the consequences without looking at the details of an individual project - which is why you should think before doing it, but be aware that you do have the right.

There's a difference between what's permitted by a license and what is polite/good.

The reason people use licenses which give you permission to fork and redistribute, even when they want you not to, is because they believe you should have that right if you need it, but would rather you choose not to exercise it without a good reason.

That's why when someone takes BSD-licensed code and wraps it in a GPL license, sometimes people get upset and say it's wrong (even though it's permitted), and other people say "but you explicitly let me do that!", followed by "if you didn't want me to do that, you should have used a different license!".

The latter people miss the point. Spelling out everything in a license cannot provide the subtlety of granting essential rights when needed while asking for thoughtful, mature discretion in deciding when to use those rights.

Yeah, it's permitted. Those rights are important. Developers choosing FOSS licenses know that; they aren't stupid. If you need to fork and redistribute, do it. Go ahead, it's permitted for a reason. But it's not always kind, smart or good for the community, so please be thoughtful when deciding whether doing it is a good idea in any particular circumstance.

If you do take a for-pay app which is under FOSS license and make it available gratis, perhaps because you know people who would benefit (perhaps it's too expensive), then at least have the courtesy to mention where it is sold by the developer, so that people receiving the app can make up their own mind about whether to reward the real developer(s) / fund it's continued development.