Certainly a fair point, but it also costs a lot of person-hours to patch up that infrastructure's security and trace who's placing the calls when one could just choose not to do this fraud in the first place. I am not old enough to know whether carriers also charged each other back then, but at least nowadays it could also incur charges for the originating party; costs which the caller isn't covering

Toying with the system, learning how it works and finding what you can make it do, there's a certain art to it and I'd encourage anyone to at least tinker with the systems they own (and everything else within reason and ethics), but there's two sides to nearly everything