It's pretty open-and-shut that the jammers are illegal; but I suspect that the main concern is smacking the ones that get too blatant about it hard enough that they don't become normalized.
It's news to no one that a talented tinkerer(or mercenary no-name Chinese OEM) can cook up reasonably elegant jamming gear; and that Harris Corp and friends are already cooking up worse; and that a non-talented tinkerer willing to just throw enough power into a spark gap can cook up an inelegant jammer; but so far the(spotty; but occasionally very humorless) enforcement seems to have made things like 'owners of restaurants and theaters routinely installing jammers' something that just doesn't happen. That is the sort of thing that would really, really cause a customer support/quality of service nightmare for Team Telco; and it mostly hasn't happened. I'm honestly a trifle surprised at how effectively they've held the line, given that Customs doesn't have a chance in hell of verifying what the zillions of pacific rim widgets crossing the border actually do.
Also somewhat surprisingly absent(but probably good for some serious sentence enhancements were it to happen) are reported cases of jammers being used to suppress 911 calls during the final approach to a victim during a robbery, mugging, etc.