This has been a long known issue as exhibited by Kai+Furnace combo, where furnace basically gets 2 turns of revenge because Kai is a fast hero.
this issue has become more evident (though probably has always existed) with stonefist's epic, and the construct weekend.
with Stonefist's taunt, because the buff is round-based rather than hero based, he effectively has a 3 round taunt on a 5 round cooldown. (this is the same with phemus, though, equally little used hero, so never really been bothered.
if this is indeed intentional, please understand that this creates a VERY undesirable attribute as a "tank" for PVE purposes because not only are you saddled with a slow tank that will let an entire round of attack happen without tanking, but also leaving you exposed for 2 rounds vs. a normal speed tank that would only have one.
the only time the slow tank would be desirable would be in a dungeon where every single mob is slow speed so that you would at least get the true 4 rounds of tanking that's suppose to happen.