Actually SaltyDog, they can't enforce the use of WAG bags because there is no Forest Service Order requiring you to use one. It's "voluntary" not that you have any reasonable alternative since they removed the toilets, but they don't have legal authority to do anything special to catch or prosecute anyone. They could get people for littering but that's about it. I don't think they want to open the legal can of worms they have created.
Back in 2004-06 the former Whitney District Ranger Garry Oye circumvented the NEPA environmental review process when he implemented this WAG bag program. He ignored the impacts stated in the Environmental Assessment and he ignored the public comments, which were running 141 against WAG bags to 19 for WAG bags and implemented this program without a decision document required to complete the NEPA process. If they ever had to defend what they did in court, they could be defeated by a first year law student.
These enforcement ideas come up over and over again to solve the human waste problem with poop police. Sorry, think of another idea, say maybe toilets. Another alternative might be to install collection receptacles where people could do their business in the plastic bag, but have a choice whether or not to carry the stinky mess back to the trailhead or deposit it into a storage container. Now that would be a truly "voluntary" program and it would probably work as well as toilets.
The facts, references, and debate are all discussed in detail at
Solar Toilets vs Carrying Wag Bags 
