the revenue from crypto-related businesses is potentially huge.
they don't give a shit about that revenue. Facebook is making the same amount of money with or without crypto related ads on their platform.
and i believe the reason why they changed their policy is because of the social pressure that was put on them after they banned them. but if they really unbanned everything apart from ICO ads then i thing it is a good thing.
At least until facebook have been getting a big scandal with US. I remember there are some canadian countries have created a petition to lift the ban for ico advertisement. If they are all banning facebook from their countries and it can become another big disaster for facebook itself and that's real reason behind it i suppose.