Umm...that's not how the legal system is supposed to work.

Sure, if you cannot catch a gangster actually killing someone but you get them on tax fraud that is a real violation of the law still. In this case, however, Mesaba basically was just a bunch of schmucks. They told the CoEx plane to show up and that they'd handle it. On arrival they didn't handle it so well. And now they're being fined. I get the Co/CoEx fine as there was something published that they didn't adhere to.

Sadly, there is no law against being a schmuck. There isn't even a published guideline on Mesaba's site that says they won't be schmucks. Yet the DoT is enforcing against them anyways on those grounds. Hence my slipery slope claim.

I truly believe that Mesaba shares some of the blame for the outcome of the event, but I'm not sure that they are really legally culpable.

As for the suggestion that CO could have had valid, tested procedures, you don't really know that they didn't. They had a process for finding an acceptable alternate landing site and arrangements for ground handling there. Surely every airline cannot be expected to staff every alternate airport with local crews, right? There was definitely a breakdown in the process, but that doesn't mean that CO doesn't have a process in place. After all, they are able to handle diversions with some regularity without stranding customers.

Both carriers are culpable to some extent and CO is definitely on the hook legally. But that doesn't mean that they weren't trying to prevent the situation or to undo the mess while it was going on.