Some do, some don't. Some even reload the page when you try to view source (!). Ideally, it should pull everything from its cache, unless the cache has been set to zero. As to why, it's just the way the browser coders decided to do it. I can't think of any obvious rational as far as printing except for the fact that it might be easier in their code to call the page then to retreive it from cache. I work on Mozilla, and this bug came up a while ago - Mozilla will be smart enough now to know when to load from cache, and when to reload the page.

FWIW, I think the first answer that chromatic gave is the best solution (not returning a form, just an html page.