Fortunately most national parks don't allow bikes of any sort on the actual trails. They mentioned Acadia, but those "trails" are more like MUPs. Wide, packed gravel, few turns, I've ridden them on a road bike with 23s. I don't think ebikes will damage them any more than the horses. True MTB trails in national forests and recreation areas will be more of a concern. If they allow horses, they may as well allow ebikes. Also, even though they don't have to work going uphill, I don't see ebikers developing the skills needed to negotiate MTB trails.