BitBucket's pricing is at $1/seat/month in the worst case though (assuming you match one of their intervals there). But yep I agree, Github is surely significantly more profitable than BitBucket.
Seems like Bitbucket was acquired by Atlassian for two reasons:
1. Free DCVS hosting as a free perk to existing customers, with tight integration to existing Atlassian products.
2. Free DCVS hosting to non-customers, with the hopes of converting some to the Atlassian suite.