Do you have evidence of the "Real Reason" scenario as opposed to the Occam's Razor scenario that (a) Philips really can't guarantee the protocol works with any lights they don't explicitly test against and (b) the reason some manufacturers can slide in at a fraction of the price is that they're cutting necessary corners in either hardware or protocol compliance? ;)
I think most users would see open connectivity protocols as a feature, and being unable to connect whatever lighting fixture they wanted to the Hue hub as a deterioration of their "consumer experience".
Connect some dodgy 3rd party lamp that is non-compliant, or worse, infects your system w/ malware and breaks your products? That's a bad consumer experience you've brought upon yourself, knowing full-well the risks. If anything, it'll make you an ever more loyal Philips Hue customer once you shell out another $200 for another starter pack and never touch the untested 3rd party stuff again.
I'd be very surprised if this was anything other than the classic case of - build a very nice garden, then wall it off, or else go broke.
