This is where the good ethics of your average employee come into play. Most people play by the rules and don't abuse so even while the employer can have the 'right' to dig into things, they don't usually need to.
And when they do need to that person's 'crimes' often become the focus and the defense of 'you had no right to snoop into the computer you bought me to work on to find those 89 pornographic photos I downloaded over your network' ends up not holding water.
The focus becomes so much on the crime that the methods get glossed over.
So as long as most employees don't 'ask' to be snooped into, most employers can either pretend that they don't or they actually don't snoop, and the rules never get challenged.