When donors gave money they entered into a contract. If Go Fund Me unilaterally changed the terms of that contract after the fact they are guilty of breach of contract.
If the terms of the Go Fund Me contract prohibited protest fundraising but allowed this fundraising and then cut off the fundraising, Go Fund Me is guilty of fraud.
If this fundraising was prohibited by the Go Fund Me's terms of use why did Go Fund Me allow the fundraising in the first place?
If Go Fund Me claims criminal acts negate the contract, they will have to explain why Black Lives Matter was funded during government imposed curfews, riots, destruction of property, looting, and arrests.
If this is left unchallenged, any business can confiscate anyone's money at any time for any arbitrary and capricious reason.
The damages are not [just] contract damages. What GFM did was fraud, wire fraud and under the consumer protection laws of most states a deceptive business practice. It is also a subversion of the free speech rights of those who donated funds to a legitimate cause that they believed in.