You're not legally obliged to pay for anything, if you don't want to buy what's on offer.

You seem to have forgotten that when you buy software, you're not just buying code (which is the part you can get elsewhere if it's FOSS). You're potentially buying branding, certification, testing, support - things which some people value and prefer to pay for.

Aside from those intangibles, yes it is basically a form of donation.
(And I'd be delighted to see a Magnatune-style "choose your own price, and it can be zero" option).

There is however a down to earth reason for not calling it a donation: Corporate buyers often cannot pay "donations", but they can buy something even if that thing (or the code at least) is available for free elsewhere. That's why you often see things for sale at several prices, where the differences are marginal and the significant difference is whether it's called a "Corporate Subscription", "Premium Subscription" or "Poor Person's Subscription" or something :-)

If a person says, truthfully, "I spent 2000 hours working on this, full time every day for 12 months; it cost me $30,000 in personal savings to fund myself (rent, food, etc.) doing it; feel free to give it to your friends but I'd appreciate if you don't publish free competing versions prominently all over the net without a good reason, but if you have a reason of course you have that right", do you think that's morally dubious - assuming it's true?

Would you really have a low opinion of someone who spent that much time, personal energy and their own money on a project, to make you some open source software, for which they give you all the freedoms but ask you to not jeopardise their ability to continue if the only reason you have is "because you can"?

Do you think it's wrong of Red Hat to sell Red Hat Enterprise Linux? Even though they pay the full-time salary of half the Linux kernel developers, a fair chunk of GNOME developers, and everyone knows you can get a free version, compiled from the same source, from CentOS?

Red Hat don't stop you getting the free one built from the same source from CentOS. They don't try particularly to hide this fact (though I admit it's not found in their advertising). They don't pursue anyone for it, or ask them to stop. In fact, they try hard to satisfy their GPL obligations and community expectations by making it easy for CentOS to do that. Red Hat are widely regarded as good citizens in the Linux free software community because they consistently do these things.

You may know there are several important but small differences between RHEL and CentOS: Testing, certification, and branding. To some people, those are worth paying for. To others, they don't need that, but they want a reason to pay Red Hat, so that their interests are developed for. Others are happy to use the CentOS community-built version, which is free. I myself have used both, depending on circumstances.

Those differences would probably apply to selling free software for the N900 too. Just because you can take the source and build your own version, doesn't mean you're free to put the original developer's personal graphic signature or their "I have tested this build" seal of approval on your version.

You may find a situation similar to Debian's Iceweasel vs. Firefox, or a situation that is similar to the difference between free users and paid users of shareware, i.e. access to extras like personal support from the authors.

It does not have to be uninformed. You seem to think that misinformation is the only way it could be done; that is incorrect.

If people were sold something without being told that it is based on FOSS, then I'd agree that is not ethical.

If people were given something for free, without being told who really wrote it or deleting what the people who wrote it say in accompanying READMEs, I'd call that unethical too.

Sounds to me like you have a particular idea about how these things are done, and aren't particularly informed yourself about the various ways FOSS is sold ethically (at least, by some people's standards ethical). Have you ever read the GNU Manifesto, or the FSF's position on selling free software?

You seem to believe that the only way a person would sell FOSS is by tricking people into it, that nobody would willingly pay for it if they knew they could get it (or something built from the same source but not certified by the original developers) from someone else for free. That is not so.