I can't find it now but there was an article posted a while ago that explained the existence of just such rules back when the very first mobile phones used analogue transmission, which of course was easy to intercept. So the FCC forced radio manufacturers to prevent users being able to tune to those frequencies and additionally specified that manufacturers be responsible for making this limitation very hard to defeat... rather than fix the original problem of course.

Those rules exist to this day but have no relevance. However I suspect there are other similar but slightly more sane rules in play since modems are now just software defined radios that can be made to do nearly anything you want given the right access - i.e you can cause a lot of trouble, even if you don't intend to.