I think the reason for this is that it would be too easy to stop barbarians spawning in a very large area when utilizing hawks.
This is true, but when you are on an island it's seriously just this side of unbearable keeping a bunch of suppression units wandering around to soak up Beastman stacks. You can save yourself a little heartache by waiting for Adepts with mud golems and then just killing the Adept; I think so long as the golems remain that "slot" stays used up, yes?