The main issue with dogs in wilderness areas off leash is that they harass wildlife and trample understories, which can be home to vulnerable species. The wildlife harassment thing doesn't seem like it's a big deal except if it's every third hiker with an off leash dog then these poor animals are forced to live in a constant state of terror.

As long as they're leashed and kept on trail it's not a big issue, and very very good for them mentally