We've never heard of a bank that tells people what they can and can't do with their money.
Try using your credit card at an illegal (/unregulated) physical casino.
This has to do with basic US laws. Coinbase is a registered company and hence has to follow the laws they're under.
This means
FinCEN having an eye or two on them, and in conclusion also all coinbase users.
Online Gambling is a big red flag in the US (or at least most states, I'm not an expert on their laws, rather sharing what I've come to know over the time),
there's a reason why Primedice and some other sites block users from the States.