Transocean are a US company too, although they moved their headquarters to Switzerland for tax purposes a year or 2 ago.
There is an old US law covering shipping liability dating from 1851 that limits payouts to $27 million, and that's what Transocean are trying to hide behind.
What's worse is Transocean have already received a $400 million payout from their own insurance for the loss of their rig.