Think of it this way. What would happen if Google, facebook, or twitter hotlinked to a large file on your site and you received no revenue from it? If that didn't make sense, the cliffs notes version is that you'll end up with a gigantic bill from your hosting provider and a DOS'ed site. That's why it's bad form to hot link without permission. It's a rule that's developed over time in response to stuff like this. Also, it's a two way street. Many sites will black list referrers from sites that do a lot of hotlinking as its a really common spam tactic.