It's totally reasonable for Google to decide they don't want to spend the resources to support WP8 due to low usage. But here they were taking an apparently already working (even if not officially supported) site and breaking it. That's a bit less reasonable, but it could admittedly still be a technical issue.
But that clearly can't be the case for the GMail and Youtube issues. Google still continues supporting the Exchange sync protocols for business users, so there's little technical benefit in disabling it for another class of users. Likewise Google is under no obligation to make a native WP8 Youtube app -- but given they've given partners API access to Youtube in the past, clearly there's no technical reason why they couldn't also open up the API for MS to use. In either case there's no plausible way the low market share of WP8 could be the explanation.
Now, of course both of these cases could have totally benign explanations. Maybe the licensing terms of ActiveSync make it undesirable for Google to support it for free users in the future. Perhaps MS and Google can't come to a reasonable agreement about Youtube API licensing terms, and the real villain of the story is MS for trying to now score cheap publicity points with it. Or maybe Google is trying its best to smother WP8 in the cradle. And you can't ignore these other data points when looking at the motivations for these petty Maps changes.