The obvious flaw in this argument, outside of what others have already mentioned (doing that is independently illegal), is that you don't need to modify an official device to do something like that. Building a radio jammer out of fundamental component parts is not a complicated affair.

It's like arguing that people shouldn't be able to modify their home batteries because they could use them to purposely damage the power grid. That's kind of a ridiculous scenario to worry about when the same infrastructure is already vulnerable to utterly pedestrian physical sabotage. It's a facile rationalization for imposing restrictions that have an ulterior motive.