Re: Minor edit...
By the American safety model - 'it has to be proven unsafe to be grounded' rather than the 'it has to be proven safe to fly' precautionary model adopted elsewhere - the FAA would probably have had their arses sued off them for billions by Boeing if they had dared to ground it.
That, surely, is part of the whole problem here. It should be the FAA's duty to ground the plane if there is reasonable cause to believe that it might not be safe to fly. There should be no grounds for Boeing (or anyone else) to sue them for doing that (unless, perhaps, it could be shown not only that the plane was safe, but that it had been obviously safe all along).
It really worries me that commercial entities are even allowed to sue government bodies in ordinary law courts. Yes, the state must be accountable ... but not in the same way as a business.