And then it becomes a problem of proving your users violating the GPL. So you'd have to go after each one of them, which will be incredibly difficult, and proving damages would be even more difficult.
It's an asshole way of exploiting "Wo kein Kläger, da kein Richter" (where's no plaintiff, there's no judge) since actually proving that the developers violated the GPL will be difficult, unless they have a CI system that readily documents this.