An established legal case has already defined the IndieGoGo donations / pre-orders / pledges / whatever they are called as "orders", nothing less, in this exact case.
They can denounce the project.
They can seek a refund on backer's *ORDERS*.
They can throw the project off IndieGoGo.
They can retract all endorsement of it.
They can attempt to recoup their backer's money via the courts as they promised (if they get refused, meh, that's bad luck, but they could try!).
IndieGoGo have *literally* said they would do the latter, WHETHER OR NOT the products shipped after their deadline. They haven't.
It's not that there's something explicit and simple they could do legally. It's that they have promised things and then not even bothered to do them. That's enough to make them complicit and stop people doing business with them.