Why would Apple ever invite Valve to potentially do the same to them?
I actually see it as the reverse. Valve might be going for the whole pie and want to carve out a niche for their Steam Box. Inviting Apple to the party might detract from that effort. Or at the very least distract from their main focus.
I don't think Apple wants any non-Apple store addressing their weaknesses, especially a solution as competent and well-funded as Steam.
If Valve gains Apple-user mindshare on Mac, what prevents them from expanding to iPhones and iPads in the EU, and likely elsewhere if anti-monopoly laws get entrenched? IIRC, Services is the fastest growing revenue source at Apple.