Seems to me the exchanges violated the fiduciary trust they held to keep the funds safe when they stole them and exercised the right of ownership to use them to vote on witnesses. The actual owners of the funds were deprived of their votes by the actions of the exchanges (an abstention is a vote), and now the blockchain is irreparably harmed and financially diminished as a result.

Multiple grounds for tort action to seek remedies IMHO. Sadly, IANAL, although I'm told I should be. Mostly because I argue so much.