Its human nature to take short cuts unless we are being checked up on. So in this case education about sterility of tools is not enough to ensure that doctor offices actually sterilize their tools. So the law is in place to create a standard that everyone has to abide by and reprucusions for not following the rule. You just have to look at how many times doctor offices are fined by both private and public enforcement agencies for not meeting the standard safe sterilization practices to see that left to their own devices, people don't do what they are supposed to do. Also, the legality of making this a law comes from the constitution where it says the gov shall promote the general welfare. Preventing deaths by enforcing a rule is a common way the US gov meets that mandate.
To prevent diseases and bacteria from spreading from person to person. Use of dirty tools yield infections, bacteria, and another viruses you don't want. Especially when you being operated on. C'mon, it's common sense!