Well, they aren't just telling them not to do it, they are physically dissuading them from doing so. This is probably a lot more effective than a warning sticker to not put the cartridge in their mouth.
The irony, of course, is that everyone's reaction to hearing about the bittering agent is to immediately lick a cartridge to taste it for themselves.
Have you seriously never tasted something after someone told you, "this thing tastes awful"?
But at least with such a taste left in their mouths, they'll surely not do it.