I think this is very wrong. JB by nature isn't a big deal to me and I don't care if people do it to enhance their experience. However I don't like that they feel they should do it at the Apple stores. It creates unnecessary work for the staff and give people a false sense of what they can do on their phones if they aren't the wiser. Not only that, but Apple also can also start locking down the device to where you can't go anywhere on the net except to a restricted list. This also takes away from the shopper.
The simple fix is of course to block the sites that host this JB. I also think the ones who JB those Apple store phones need to grow up. You gain nothing by doing this but creating unnecessary work for the staff. JB at your leisure on your own time. It's like going to the car dealership and taking a car for a test drive and tweaking the car before it's returned to the dealership. At least here in Texas, some dealers let you take the car by yourself for a test.