Fair enough. Leaving something functional and unsupported is not the same as artificially limiting something from working that used to work fine.
What if AMD turns off 3D rendering capabilities when an Intel motherboard is detected? You'd be ok with that then? Even if people who bought the Radeon card for it's 3D rendering were never told in the requirements that the video card will ONLY work with an AMD system (processor, chipset)? That's fine as long as AMD's excuse is that they don't want to be responsible for something they don't support even though the Radeon card gamed just fine on your Intel system for months?
Again, I said it is arguable. I can see the problem and it won't be better if a message pops up and say "PhysX is unsupported when ATI card is present, do it at your own risk!" Nvidia will have to release fixes to unsupported functionalities because that allows users to do so. "If it used to work, but not anymore, then Nvidia have to fix it." Even if it was not a supported function to begin with.
Just because it appears to work fine doesn't make it bug free. I learn it the hard way. "Don't let me do it if it was not meant to be done." You may disagree, but users are smart and found hacks even when the developer tries to stop them. The key here is, words "Hack" or "Mod" more or less mean "No warranties".
Lets talk about CPU. Mobo use to support up to C2D and there are new mobo that support up to C2Q. Some user found out that those board that are suppose to support up to C2D actually support C2Q. Well, I went out and brought myself a 680i SE with a Q6600 hoping that I can OC it like crazy. It ran fine, but soon I found out that I can't OC. Who should I blame? Some people brought a new 775 socket board before they found out that there C2D CPU don't run. Who should they blame? If you can fit a cpu into the socket, then it should work! To avoid this mindset, AMD socket is different that Intel socket and user can't mix them. Problem solved. Some happens to memory slots. Can it be modded? Yes, it will void your warranty though.
If I have derailed the course, I am sorry.
Edit: I understand your point of view, but I don't think PhysX can be done independently. For cross-over to work, the drivers of the vendors must work together. I don't know the specifics, but I do see the complicities. Having it work base on luck is easy, making sure that it works for all possible scenarios is hard.