I think that decision is based on assumption that you read the page one time and always linearly (from the top down). So scrolling up means that you finished reading the page and want to do something else (thus controls would make sense). While this can be true in some cases, it's not always true. I can open long pages and scroll back and forth (and during that, controls would only stand in the way). So why can't pulling controls be triggered by some other action?