When you summon a dremora what exactly causes them to obey the summoner and not just cut them down?

In Skyrim you can learn a spell to summon an "unbound" dremora who attacks you. However this confuses me a bit because books I've read in Oblivion talk about how summoning spider daedra is risky because they can harm their casters. Shouldn't they be bound? Considering that dremora are intelligent beings much like mortals races, even being treated as NPC's in the game engine, shouldn't they be the ones who can't be controlled?

How does one "bind" a dremora's will like that? If one can force a dremora to do certain things despite them having intelligent minds then why can't mortal races be bound in this way?

It depends on the skill of the summoner. Even a Scamp can get out of control if conjured by an unskilled caster (which happens in Morrowind). I don't know the exact mechanics.