Just curious

[Penguinfan]

When changing tables in a tournament whey is it OK to be moved into the Big Blind but not the button? I know this has been the practice forever, but there has to be a reason it's OK to get screwed by posting Big Blinds in back to back hands if you get oved from a seat where you just posted into a seat that has to post, but not acceptable to move into the button reguardless of what seat you came from. For that matter why can you move into the Big Blind, but not the small blind? Whats the difference?