Plenty of arguments in this post are talking about kids being able to run and play. But this is not the case described in the post. P.E. is not unstructured play time in which a student gets the chance to set his own limits according to how he feels.
The P.E. teacher will determine the activity and will most likely be telling Johnny to keep (skipping rope, doing jumping jacks, 5 more pushups). The teacher is unable to monitor if the child is outside of a safe pulse range and most kids couldn't tell you either. Partly because of the math involved which includes checking your resting pulse.
Schools are starting to figure out that if Johnny needs a waiver and a physical to voluntarily compete in track after school then they are certainly liable if something happens to Johnny while he is involuntarily participating in athletic training directed by the coach during gym class. Both are physically strenuous activities being directed by a teacher/coach that is simultaneously trying to supervise around 30 other kids. And that teacher/coach has absolutely no way of knowing if the child is in good enough shape to participate or when they child has hit their limit.
School has longer hours than it used to. Homework is long. And parents work more which means parents have the kids in daycare instead of playing in the backyard after school. These kids are in as bad of shape as we adults are. Monitoring heart rates is kind of smart.
On the other side, it sounds like the school is pretty much admitting some liability on their part... knowledge that the students will have different limits of how much exercize is safe. If they aren't going to buy these for every kid they can pretty much plan on being sued by the parents of the kid who didn't have one and "was too tired to do well in class after gym".