It's about making a statement that mediocrity will not be accepted. Accountability starts at the top. If ZT isn't going to hold Dalton accountable for his poor play, then why should Dalton or anyone else on the team be worried about losing their job? This team has been in a realistic position to win 2 out of 3 this year and Dalton has turned the ball over and let his entire team down every single time. If the head coach says "that's ok", then this team is done until someone else is at the helm.
Edit to add: if the definition of insanity is doing the same thing over and over, but expecting different results then I'd argue that benching Dalton is the exact OPPOSITE of insanity. We know we will lose with Dalton at QB. Benching him tells the team "losing isn't ok, we must win at all costs, nobody is safe".