This has been in practice for well over 10 years.
I will never understand it other than the obedience angle someone else brought up.
If you are unable to determine the difference between a kid who abuses the privledge and the one who has to go, you should not be in charge of 2nd graders.
Had to go through a doctor back in the day, for my child who truly had to go, often.
Happened to me in the 70’s, that is definitely over 10 years, in the same school system as this story.