Taking an emotional support animal along for free was a recent law (last 10 years?). The rules for ESA only applied to housing and air travel - I don’t think it even applied to local buses or taxis (SERVICE animals are different and they can travel on public transportation).
An old roommate had little dogs and she would not travel with them in cargo, but several airlines allowed her to have them in the cabin in a carrier, and for a fee. She was fine with that and would even fly with connections or to out of the way airports in order to have them in the cabin.
People definitely did it to get a free ride for their pets but also to keep them out of cargo. Also, there are times of the year when pets can’t be in cargo (too hot or too cold) and the only way to get a 40 pound dog in the cabin is as an ESA or service dog. My niece has a great dog, great traveler (curls up at her feet) but it is NOT a service dog and niece doesn’t need an ESA. She was abusing the system.