That's a good point, and I agree with you to an extent. But there have already been plenty of opportunities for Facebook to muck things up since the acquisition. The fact that they haven't done so in any way does count for something in my mind.
Also, I think the increasing pressure from HTC and Sony is showing pretty convincingly that Oculus would have been in a lot of trouble if they didn't receive solid financial backing from a larger partner like FB. The Vive has already pulled on ahead of the Rift in terms of offering a much more versatile tracking solution in the form of the Lighthouse stations. PlayStation VR is the sleeping giant in the room that nobody's really talking about.
Oculus is not going to be able to rest on their reputation for long and still win the VR game. VR is slowly becoming high stakes; it's not a harmless little Kickstarter campaign anymore.
If it's true that they're not making any money on the $600 CV1, then it seems to me that they desperately needed FB or a similar partner.
Ultimately, we're faced with a dilemma where the deeper we go into the Oculus + Facebook timeline, the further we move away from being able to predict what might have happened if Oculus had never been acquired. If FB does something to upset people (like requiring FB login), there might be a lot of outrage, but nobody will be able to say for certain that Oculus could have even remained competitive without FB. It won't be long before we reach a point where nobody will be able to competently know what we should be comparing FB-owned Oculus to in this hypothetical alternate timeline where Oculus was never bought.
The good news is that if Facebook does seriously muck things up there are great competitors who will gladly take your money instead. I just don't think there's any sense in condemning the Rift based on some hypothetical future wrongdoing from a company that has so far treated the platform quite well, especially taking into account the fact that we don't really know how well Oculus would have fared without the acquisition.