While similar to both currency and commodity, this is one of the situations where cryptocurrency acts as more of a fungible contract than either the currency or commodity. IANAL, but it seems to me that there isn't a whole lot of law written on that. I'd argue that Coinbase expended its fiduciary duty by warning their users of the fork.
Coinbase/kraken/et al., however, don't create/issue the cryptocoins, so it's probably a misnomer to call them exchanges. At best, they are brokerages.
Now say that a bunch of users decided to instruct their broker that's holding their security in trust to so assign the security to the user, and the brokerage sat on that transaction rather than perform the request in a timely fashion? I'd say that potentially counts as a whole host of other actionable events, depending on the intent of the brokerage operators. Not sure if I'd call that front running, but that sounds like a good start to investigate.