You ain't thinking like a true hero boy. The plan was to to save Todoroki from himself and his father's influence on him. Not to mention it was a tournament for the kids too show off their full strength, how would you feel if someone fought you with a hand voluntary tied your back, probably a lot more angrier if you lost and undeserved if you won. If Todoroki kept on going like this even as he went pro, then he would easily be surpassed by Deku and Bakugo as they continue to use their full potential and possibly cause unnecessary casualties that could've been saved but weren't all because he had too much pride to use his father's quirk.

Look at Midoriya's face and tell me what he did was a mistake.

That's exactly what he wanted. Heck, he's even showing his inner Bakugo.