There can be a difference between whose the fault is, and who is accountable. Usually, the entity you paid is the one legally accountable to you. This is not just a trivial detail: you will have no luck with the law attempting to hold the other entity to account, even if they are at fault; while you can hold your counterparty to account even if their action was dictated by someone else; they have to act correctly with you- it's not your problem if their relationship with some other company is put into contention. This may not apply to domains, FWIW. usually, buying a domain means you have a contract with the registry too. I am not a lawyer and this is not legal advice.

This can big companies too, FWIW. Cisco were in violation of the GPL over the WRT54G, which resulted eventually in the release of OpwnWRT. Well, Cisco weren't the entity which actually caused that fault; the chip vendor was, as they were the ones who used the linux kernel without releasing the source. That is not a valid reason to violate a licence.