"If you can show that someone else was doing the same thing without being charged"

It's unlikely people would have enough evidence of high enough quality to make that case. The prosecution will just say the evidence isn't enough.

The prosecution can say whatever they want, but the worst of the existing laws are the one that are pretty easy to prove, and it's the court rather than the prosecution who makes the decision.

You'll also get some nice incentives where the government will be arguing that a particular behavior is not a violation of the law so they don't have to charge someone they don't want to, which in turn leaves you with that precedent if the judge agrees with them and then they want to charge someone else for doing that later.