If they're of any appreciable size, they're paid for by the people hosting, not downloading. You do know this, right?

Of course.... but who pays (and for what, exactly) is somewhat immaterial. You're able to post public links to files and get (effectively) unlimited downloads via Dropbox too. It was a counter-example to your Google.com/yourdesiredfile

Who pays (and why) is hardly immaterial when it has a defining effect on how the site is actually used. There's a good reason as to why Google and Dropbox can easily claim legitimacy while Mega has to struggle.