Read: http://en.wikipedia.org/wiki/United_States_v._Microsoft
I'm not trying to be snide either, Microsoft leveraged its relationships with OEMs to incentivize them to stop bundling netscape. I don't think that Google would play that dirty, but it's certainly thin ice.
that won't happen, if only because google maintains complete control over any android applications that interact with its services. gmail, voice, market, etc. are all closed-source proprietary applications that ship with most phones and are not part of android's open source code base.
integrating any g+ features into android would also mean that google would not be able to push out any updates to that code to take advantage of new features. some phone carriers/vendors release devices and never send out updates for them. at least with separate applications, google would be able to push out updates through the market.
of course, that's not to say that google won't develop a tightly-integrated application for g+, but that is possible today with android's current api and doesn't have to be a part of the system.