Google doesn't hold a monopoly over the entire smartphone market, and it doesn't have the same level of influence globally. But in the two areas where its Android operations have aroused regulatory scrutiny, the United States and Europe, Google enjoys a practical stranglehold over the mobile operating system market - thanks to Apple's non-participation and Microsoft's chronic failure to compete. It's arguable that other tech giants, such as Apple and Amazon, are better subjects for antitrust investigations, but US and European authorities are right to at least consider the circumstances of Google's relationship with its hardware partners.
I don't know if Google has a monopoly over the phone market - and neither do you, because the laws regarding monopolies are ambiguous, incredibly complex, and differ per jurisdiction and sometimes even per sector - but I do know that as far as I can tell, Google isn't blocking anyone from shipping Windows Phone devices, nor is it stopping developers from publishing applications for other platforms or even in other Android application stores, nor does it stop anyone from taking Android's code and building something that competes with it (see China and Amazon, for instance).
In fact, we should thank Google for building and releasing Android, because without it, iOS would've evolved a lot slower, we'd have less choice, and we could've even been stuck with just iOS and something from Microsoft - much like on the desktop.
That being said - I'm always in favour of keeping very close tabs on powerful companies like this, and in my view, the Microsofts, Googles, and Apples of this world should always be kept an eye on. Better yet, in an ideal world, all code in our computers and mobile phones should be open - from operating system to firmware - because of how crucial they've become to our society, but alas, that will never happen because reasons.
