Simple economics and human nature.

A 2 minute internet search shows United wants $250 to ship your dog in a crate in cargo. Delta wants $100 to bring a small dog in the cabin as a carry on or $200 in cargo for larger dogs. In addition, Delta says you may have to drop off/pick up at the cargo facility, not the pax terminal.

Or, for $65 on the internet, you can get a certificate and fashionable "service animal" vest and avoid all the hassle.

Nobody should be surprised.