I have to agree with you on mixing JSON with HTML in your response, that's just weird, but there are reasons for the other things.
The difference between the HTML and JSON response are nominal compared to the rest of the page. You'll save more bandwidth by taking away a couple of colors from a single gif. No, the real advantage here is having a degradable interface in which you only have one place where you're managing the html display aspect. Don't get me wrong, I'm not negating trying to cut down the size of a response, I just don't think that's the reason behind this.
Re: the code formatting, they're probably just stripping whitespaces to get rid of some of that bloat. Just look at Google, they're notorious for their "tiny" js.