I'm sure they disclaim responsibility for this sort of thing, since, you know, they don't want HIGH RISK PURCHASES?
And, do you know that every financial services provider ever does this?
EDIT: Yup, they do!
3.1 Coinbase may cancel or reverse potentially high-risk purchases made using reversible payment methods.
https://coinbase.com/legal/user_agreement
There is no legal basis for this thread whatsoever. Coinbase wins. Sorry!
Aww, this was the conclusion that the OP was supposed to draw after digging up the contract that they agreed to.