You can upgrade it, but you may not like the terms if you don't have a PPC or are not interested in MOS, just like noone will be able to upgrade OS3.9 while he is running on a 68k or x86-based emu...
I partially agree, which is why I said more "annoying than unjust".. but I still think that when you rely on a limited number of users for an income, it's better not to annoy them by forcing them down a particular upgrade path - that may or may not be the one they intended to take. Obviously buyers of the software may not want or (as you point out in other posts) need to upgrade - but for myself.. if I fork over the cash for a product, I like to think I can stay with it for the long haul - not just for my immediate needs.
For example it may not be needed under OS4, but it could be that it's a more efficient set of drivers, and have more options available for the card you're using (fully functional OpenGL for example), and therefore although you may not need it, it would be an improvement over the OS's generic RTG offering.
However you would be right in saying that just because you buy software it doesn't "entitle" you to an upgrade path - the only morally correct thing for a company to do is to provide patches for major bugs (which I see you say are available).