the problem with leaving a pet in the car or pickup while viewing a sight such as Old Faithful geyser or Yellowstone falls, etc. is that it gets hot there in the summertime and most of the parking lots are unshaded.
you can leave your pet in your rig but you have to watch your time away. our dog can go several hours without a nature walk.
there are lots of shaded picnic/rest stop areas you can park and walk your dog.
one major reason you cannot take your dog on a hiking trail is that dogs are BAIT for predators... and once under attack, the dog can out run you!! but not the coyote or bear or lion or wolf.