Don't search for the conspiracy where it does not exist

I think what everybody wants them to do is just commit their code to a public repository as it is written, like just about any other FOSS project does.

Sadly, not everybody. There are open-source developers who really want this and there are partners who don't like the idea at all: they want to separate development from IP issues as much as possible and one simple way of doing it is to commit everything only to internal repo (which is accessible only to people who signed NDA - this means that regular Joe Googler can not see it either). If NDAs are structured to expire when the product is actually released then you don't have IP issues at all - and this is typically as far as most embedded vendors are ready to go. Of course components which require more restrictive NDAs are just not ever released at all - but you can alter your decision at any point.

You don't wait until everything is perfect to release the code - you release every line as it is written...

Sure, it's possible to develop in the open while carefully reviewing patches so as to make sure NDA is not violated: this is how Linux-related things are done in server space. But this slows down the pace of development and required additional manpower. In exchange you get what "open source community" is to offer. Is it good trade-off or not? Well, as I've argued elsewhere the answer is: it depends. While "opensource community" is in it's infancy it's more of a hindrance then asset, but as platform matures situation changes. We'll see if Google will change it's development model when it'll make sense or if it'll wait till some kind of fork will become more popular then "real thing".