Uhhhhh.....
Steam are the only people that can issue VAC bans? That seems like an awful lot of power to leave to some rando that starts a server. Abusive admins exist, and I doubt steam would give them the power to ban from every VAC secured server on steam.
Ah, lets clarify that. Only EAC and ofcourse Steam can issue VACs based on players and admins their reports and evidence. Thats why its so important to report any activity. Admins can only ban from servers and they can request a VAC when needed, not perform one.