The situation with Instagram is a little different. My understanding is that Instagram doesn't have a public API that allows ANY third party developers to post images. Their public API only allows viewing type stuff. Instagram basically reserves the right to create full featured apps themselves and for whatever reason they don't want to develop for WP right now. This is also the case with many other companies systems. They don't all provide APIs for third party development. For example, banks do not open their systems to third party developers (for obvious security reasons).
Google on the other hand does have public APIs that have been in use by other third party apps for years, but they wouldn't allow MS to use them. This is hypocritical for at least a few reasons...
1) Google made the API, but then they complain that an app using their own API doesn't comply with their own terms and conditions. Sounds like Google needs to fix its own APIs to me.
2) Google only complained about the MS YouTube app while ignoring all of the other apps that work the same way and are available in Google's own store.
3) Google states that they won't build a YouTube app (or any other apps) for Windows Phone because the user base is too small for them to make any money through ads. Google also claims that they aren't giving MS access to an existing advanced API that would support ads (and therefore meet the terms and conditions) for the same reason. However, when MS releases it's own WP YouTube app Google complains that they are losing money because the app doesn't show ads. Losing what money? The money that they said was too trivial to bother with in the first place?
Having said all that, I do agree with you that MS needs to be more aggressive in pushing Windows Phone. There are signs that they are starting to do exactly that and it's quite encouraging. Each situation might require a different strategy though.