Because like Brannew said, they think he
- rivals Shanks
- Masterminded Impel Down breakout
- Was part of the Roger Pirates
- Has a influential underworld organization which came to power once Doffy was arrested
- Was a former Warlord
They "think" he's a VERY strong Pirate which is why he was invited into the Shichibukai in the first place.
I'm not arguing in bad faith, that is the only possible explanation as to why Buggy has a lesser bounty than his own subordinate - because the Marines haven't given out updated bounties yet and just showed us the bounties of Buggy & Mihawk after the Warlords were disbanded.
The thread is definitely on it's way once the chapter is out though...