They can if their device is still capable. My 2006 iMac hasn't run the best since the upgrade to Lion (Tiger, Leopard, and Snow all ran fantastic) and upgrade to the latest iPhoto. Gave my system all sorts of problems, so I understand why they would drop support for it. Someone who purchased a Mac Pro probably invested for the future and would expect a 2007 Mac Pro (unsupported) to be on par with a 2007 iMac (supported).
Definitely a most reasoned argument and I agree.
It's one thing to expect some sort of compatibility so long as the hardware can smoothly handle the software. However, I think that it is unreasonable for anyone to expect their machine to run the latest version of the software regardless of how old the hardware is. This is the reality in the world of technology.
As far as a 2007 Mac Pro goes, I believe that it's due to the 32-bit instruction in the chipset that's responsible for the lack of support and Apple's reluctance to spend the time to rewrite the code for the chipset. Keep in mind that even after the PPC to Intel transition, Apple was still making a big deal with transitioning from 32-bit to 64-bit. And I believe the 2007 iMac chipset took advantage of full 64-bit that the Mac Pro didn't. I'm sure someone here knows more about that than I do.