If Valve lock out the rift from their games, it just reinforces the exclusivity, lockouts and polarisation. Means less sales through steam, less profit for valve. If they lock out the rift, will they lock out all the other HMD that will surface in the next couple years? That will likely stunt the VR industry given how central the steam platform is to games distribution. Thats not what valve want. Valve want it to grow rapidly and sell as many games on their platform as they can. That means supporting as many HMD's as possible. Which entices developers to use the steamVR platform as it will reach the largest audience, which entices HMD makers to support steamVR... etc.
So I really don't see how Valve is too concerned about this. If facebook locks down the rift so that it can only play rift games, more fool them. Many people just wont buy a rift.
Even if the Vive fails in the long run, there will likely be many other HMD's that will work with steamVR. Valve can still make a nice profit from all the VR games sold on steam.
