That would come perilously close to tortious interference. They would have no standing to do so, any more than withholding your funds because you didn't pay your rent.

Their standing would come from the fact that they had to refund the indie developer and therefore are now in need of funds to cover that expense incurred because of the publishers actions.

Any competent attorney (and realize that not for one moment do I have any sympathy for the publisher, who sounds like a complete garbage person) would turn around and say "You had no obligation to do so, and your desire to do so for PR purposes or otherwise in no way creates an obligation for me to reimburse you".

And legally, they'd be entirely right.

Couldn't the developer contract with Valve to allow that to happen?

the contract is almost certainly with the publisher, not the developer.