I think it's a similar reason to why people visit the zoo ;)
C4 isn't paid for by the TV licence, that's paid for through advertising crap like crazy frog ringtones
, although I am disappointed that the BBC has followed the pack and gone down the reality tv route to such a degree. They should be innovating in the areas they know best IMO.
Having said that, 'reality tv' has quite a broad spectrum, and some of the more documentary style offerings have been very good, 'faking it' and 'Lad's Army' being a few of my favourites.
I'll watch BB until I get bored of it (which I know I will), and won't be upset because it's the last one. It's had a good run - time for something else.