Q: Will Coinbase keep UAHF coins for itself?
A: No.
Coinbase does not intend to support or interact
with the UAHF chain. If this were to change,
Coinbase would make those coins available for
customers to withdraw, not keep them.
It's way more likely that they will change their policy after angry holders aren't allowed their coins. The more expensive BCC gets, the stronger the incentive for those exchanges to cave or cheat. Regardless, most exchange's hot wallet addresses are known. The second exchanges cheat, everyone will know.
This is either a political signal that they are still on team Segwit or it's a very risky bet that BCC will die, made for the purpose of saving a few bucks in administrative costs.