The title says it all. Kings should be leaders that dictate how the voting goes and should show their alignment based on how they handle that, so why should they be able to abstain on voting? They should execute by default and chose if they pardon, but have no option to select neither.
An interesting proposition is that this could count for all players. It’d make it less easy to spot a reaper game for one and people cannot stay passive anymore, but have to actively show their opinions.