Laws "against murder" != laws "to prevent murder/catch murderers". Nobody anywhere is challenging the laws again child abuse. What they are challenging is the

Laws "against murder" != laws "to prevent murder/catch murderers". Nobody anywhere is challenging the laws again child abuse. What they are challenging is the laws attempting to decrease it or catch people doing it through means that don't actually work and harm other important things. To use your example, it's like writing a law "against murder" that's written in a way that actually doesn't make murder illegal and also bans seat belts. In fact, this "scope creep" you mention is exactly how laws work. First you make murder illegal (done), then you start looking at the common causes for murder and find ways to fix those.