Airlines are under the ACAA, which is the Air Carrier Access Act - a codified version of the ADA for airlines. And until now it did treat ESA's just like support animals. You had to let them on. Couldn't deny them.

Also, many states and local governments went beyond the ADA and required places of business to cater to ESA's - so even though the ADA didn't explicitly require it, didn't mean that businesses were off the hook.