Why not? Anybody can make a homemade shotgun with standard tools and hardware store parts, but very few people do. Very few people simultaneously have both the technical know-how and the inclination, therefore gun bans are generally effective by simply banning the easiest and laziest way of getting guns (buying them.) Even people with overt criminal intentions rarely make their own guns.

I think the same will likely be true for legally-enforced client side scanning. It is already the case that few people simultaneously have both the knowledge and inclination to "jailbreak" their phones. Throw in stiff legal penalties for doing so and even fewer people will do it. A few people still will, but if most people don't then the ban will still be effective even though it's possible to squeeze through the cracks. In both cases, instructions for circumventing the law may be found online by anybody that cares to look. But most people won't.