Why? Why unless you are in a position that you know is an on call 24/7 should you be obligated to be reachable when off the clock? Now if an employer wants that as a condition of employment that is fine but to just decide out of the blue that employees should be at your beck and call for any reason at anytime is nonsense. Firing employees because they have a life outside of work is wrong.
The primary reason is GOVERNMENT has no authority to control private business.
Secondarily, salaried people are paid to get the job done. Sometimes that means working a lot more than 40 hours a week.