You're assuming these fights are willful one on one matches. There have been more than several occasions where Superman is going for killshots because he's under mental or magical control or some evil influence.
Whenever that happens Bruce barely survives or gets saved by someone with super powers. Or it turns out that Clark was still holding back because of some Kryptonian mental blocks. Or the villain didn't know how to use Superman's strength. Or the villain just wanted to fuck with Batman instead of kill him.