To keep them from hurting themselves! and besides insurance would be through the roof, man if they didn't! Not to mention the law suites they could file in the event of an injury! Dude your question rocks!
If you have ever seen their helmets in an exhibit you would know that it did not offer much protection. The true reason is to keep the pilot alive as he dives through heavy fire towards his target.
This question is hilarious, so I feel kind of dumb giving a serious answer. I would guess that it was to prevent them from getting knocked unconscious in the event that their stupid heads hit the canopy while trying to evade the aa and fighter cover.