Well, I feel if someone had wallet access, and saw funds weren't there, they should be liable, yes. If with nothing else, at least for failure to report.
That being said, Coinbase isn't being sued because they saw funds weren't there. They are being sued because they had faulty KYC/AML, knowing Vernon had scammed people, and still allowing him to liquidate. They turned a blind eye to it. That's what the suit claims.
To be fair, if Coinbase did their job properly, they would have locked accounts, or not allowed it, meaning a good chunk of funds would still be there, or at the least, reported earlier, giving feds an opportunity to seize computers or servers early, maybe saving information. Who knows though, it is a what if scenario. Maybe if it was reported, feds still wouldn't have done anything in time, maybe they would have.
I agree, it doesn't directly catch or even help catch Vernon, but it does help make investors whole.
It isn't ideal by any means, but at this point, I do not believe any prosecution will reach Vernon unless he comes back to states, but at least it helps some people get money back.
Maybe if we're lucky, china will look into bitebi9 and get him with something there.
If the hacker isn't Vern or Cryptsy insider - most likely a rogue federal agent - like Shawn Bridges