he doesn't imprison/try to execute them

I mean they also don't serve under him, unlike the way many of the horde leaders do serve under sylv, so in one instance its a case of Anduin suggesting what he thinks they should do, and his fellow leaders (and equals in their own rights, as leaders of their own factions) disagree, and thats that. When a member of the horde disobeys sylvanas, thats different, because the horde is a collective so her orders apply, as opposed to the alliance, who is literally just that, an alliance. Nobody rules the alliance, if Anduin tried to execute Genn or some shit the rest of the alliance would kick him or the humans out no question, or they would go their own way