Bob Crow, like any other permanent union official is elected by the membership.
He gets well paid so that he cannot be got at by anyone who might want to influence him.
This good salary also ensures that he can concentrate all of his time on union matters and means that he does not also need to have another job in order to make ends meet.
Apart from the people who have an instinctive hatred for unions, if one is prepared to look closely, with an open mind, one will see what unions are all about.
If employers were altruists there would be no need for unions but as they are flawed individuals that are under some pressure to get the most out of the workers while giving the least, unions are essential for ensuring fair play.
Without unions we would very soon go straight back to the bad old days of the "dark satanic mills".