I suspect there's some crap reporting going on here.
Campbell can't have been expelled for voting LibDem, since no one can know that he did.
He can be expelled for encouraging others to vote LibDem, I imagine.
Be curious to know what would happen if he said that he claimed to vote LibDem "for a laugh" ?
I'd also be curious about the legality of taking any action against someone based on their statements about how they voted ? I would happily imagine some Victorian (or American) factory boss trying that sort of stunt. But presumably the Labour party rules are formulated so as not to invite legal challenge.
However, just in case I needed it, I'm grateful for the suggestion from Labour to vote elsewhere. Message received loud and clear.