It's not that Google prevents you from getting your data, but as you develop you start to lean on more and more App Engine specific services (e.g. Datastore, identity, images, task queues, etc.) just because they're so convenient to integrate with. You also have to engage in a lot of funny little hacks to deal with App Engine idiosyncrasies.
The next thing you know, you've got 50,000 lines of code and the thought of porting it all to another platform with an identical topology is hardly any more palatable than the prospect of rewriting it all from scratch (since you've invariably accumulated warts you'd like to refactor anyway).
App Engine is great for MVPs and toy projects. And it can be great for large scale projects, but you better be sure you'll never want to run it anywhere else...