It's actually not that hard to prove. For example, PSN now has dynamic pricing for their games which can vary quite wildly and all it takes is a small number of consumers with price differences to prove it. The same is true for grocery stores or whatever else.

Enforcing it is another question though and you're right that companies will likely just accept the fine. It's all the more reason why this sort of thing needs to aggressively be legislated against and denied.