Apparently in the US people are so damaged that they are permitted to take their "emotional support / psychiatric service animals" on US internal flights. From American Airlines site:
Flying in the cabin
Fully-trained service animals and emotional support / psychiatric service animals may fly in the cabin at no charge if they meet the requirements.
1 emotional support / psychiatric service animal per person
Animal must be a cat or dog (trained miniature horse may be permitted as a service animal); 4 months or older
Animal must be clean and well-behaved
Animals must be able to fit at your feet, under your seat or in your lap (lap animals must be smaller than a 2-year old child)
If the animal is in a kennel, it must fit under the seat in front of you with the animal in it
Emotional support / psychiatric service animals cannot:
Be seated in an exit row
Protrude into or block aisles
Occupy a seat
Eat from tray tables
If your animal doesn’t fit within the allowed spaces, you may need to:
Rebook on a flight with more open seats
Buy a ticket for the animal
Transport the animal as a checked pet
Emotional support / psychiatric service animals must be trained to behave properly in public and they won’t be permitted in the cabin if they display any form of disruptive behavior that can’t be successfully corrected or controlled, including but not limited to:
Biting or attempting to bite
Jumping on or lunging at people
Emotional support / psychiatric service animals must be in your control at all times by leash and / or harness.
If this behavior is observed at any point during your journey and isn't corrected or controlled, the animal will be considered a pet and all requirements and applicable fees will apply.
Emotional support / psychiatric service animals
Emotional support / psychiatric service animals assist individuals with emotional, psychiatric or cognitive disabilities. Specific training isn’t required for animals to meet this classification.
Only cats and dogs are accepted as emotional support animals. Advanced notice and approval is required to bring an emotional support / psychiatric service animal with you in the cabin.
Trained service animals
Trained service animals have been specifically trained to perform life functions for individuals with disabilities, including but not limited to:
Dogs and cats are accepted as service animals; miniature horses will be evaluated on a case-by-case basis.
We encourage advanced notice for service animals, but it isn’t required
As the requirements for transporting each type of animal differ, our employees are trained to ask certain questions to determine the classification applicable to your animal.