Also, you have to understand what Open Source means. To use the Android code they have to comply with Open Source rules and release source code They can, however, keep their own software (Sense, Blur, Touchwiz) as proprietary. They only have to release the code that is open-source (including changes they make to it).
Hardware is not open source. That is the justification for locked bootloaders, etc. Google can't force the manufacturers to do this. They could, if they wanted to, exert control by withholding licensing of Google Apps like Gmail, Youtube, Market, etc. that are also not Open Source. This would be a very un-Google thing to do.
Instead they keep a top tier dev phone that is easy to unlock for those of us that want that option easily.
The other thing that is important to remember is that Google isn't a technology company, nor do they (really) sell products (even though they do a little bit). They are an Advertising Company. 97% of their enormous earnings come from advertising. Having ridiculous numbers of Android phones with Google Ads running in their main apps in addition to iPhone, Windows Phone 7 and Palm devices that are, many of them, being used to to do Google searches and run a myriad of Google apps puts eyeballs in front of their ads. This doesn't even take in-app ads into account. There is no business reason for Google to care about hardware being locked down. They might require it on Nexus devices because they believe in the full concept of open, but they aren't going to pressure manufacturers to do it on their devices. I wish they would, especially after being a Milestone owner, but that's the reality.