Yeah, that's the real rub/conflict of interest for them.
Most legit users just use it intermittently when they need to get big file back and forth to colleagues etc. I use it or megaupload a few times a year when getting big data files back and forth to colleagues when working on a research project etc., but I've never remotely had any need to pay for it.
I'm sure there are some who do that kind of stuff more often and pay for the service, but I do expect most of it is people paying to pirate stuff. Thus the site has a real conflict of interest in being expected to go aggressively after pirates when the pirates are probably their main source of revenue.