As a former college student myself, I respectfully disagree.
The issue here is largely one of respect.
In a sense, any interruption has a compression wave effect on whatever is going on. Think about how one car slowing down or changing lanes inappropriately can cause a long delay on the interstate, for instance.
I do not think that it is respectful of other students (particularly those with “ADD” or “ADHD”, who may get completely thrown off track by a call or text session) or the professor’s time and talents to use a phone in class or get up and disturb it for any light reason. We learned in church and elementary school to take our restroom breaks prior to the beginning to avoid just that sort of interruption.
As an adult student, I always alerted my professors to the fact that I could have certain work/family/legal calls that were always going to buzz regardless of my phone status - in an emergency only - and that only this would limit my compliance with their policies.
You are right, students are adults and pay for their college credits (some of them, most are on the dole from daddy or Uncle Sugar) and so have the right to “use [the service] as [they] see fit.” However, it can impact more than just “miss[ing] something” for that student, so they should accept the responsibility for that grown up decision to waste time and money for themselves and others and not complain if there are consequences which could impact their grade or subsequent time and money when they have to retake a failed course. Adults get to learn from their mistakes, too.
So what if I have to get up and take a piss at the movies....do your standards apply there as well?