It’s actually an FCC regulation that requires it, so it’s not a courtesy as you say.
First of all, if you've ever been to an event with hundreds or thousands of people (such as a parade or a rally), you'll notice that even though you have "full bars", your service is degraded beyond usability, and you'll receive text messages hours late.
But in this case, the issue is that people's phones will be trying to connect to many different towers in short succession, as the plane travels.
> Instead, it’s interference caused by altitude and the pinging of towers.
I don't believe you. Regardless, I have never used airplane mode on an airplane and I never will.
I guess I'll continue to ignore that law :)