There's nothing wrong with having a monopoly. What is, however, illegal is to abuse your monopoly to keep and extend your monopoly to other domains.
Btw Google / Android isn't near anywhere a monopoly in the smartphone / tablet market.
What would be illegal would be, for example, Google using its search engine to eliminate any mention of the word "Apple" and of the "apple.com" domain or, worse, only allowing access to negative criticism of Apple products. But they aren't doing that (at least not that we know of).
On the contrary MS has been officially judged has being a monopolist abusing its power to try to keep and extend its monopoly. Both in an US judgement and in an European judgement.
Heck, even selling the Xbox at a loss for years and years while living on monopoly money is a dubious way to enter a market (but personally I wouldn't mind if MS faded into irrelevancy and had only its Xbox division doing fine ; )
