By having root rights, you can mess with the system's core files. If companies start selling rooted phones, curious people who don't know much about the system (noobs we may say) may start exploring and mess it up. And after messing with the system, they may go to the company to get it repaired. They can also brick it. Now why would companies want to replace or repair phones messed up because of the user's fault?
Sent from my Desire HD on steroids by synergye