for the same reason that advertisers bid for their own keyword searches even though Google would place their result at the top of the page anyways - because other companies will bid their way to the top otherwise.
AFAICT it's not legal. There was a single (successful!) study done but the drug was never put up for approval as a veterinary medicine. I believe they were looking to use it for humans and it got stuck in limbo there.
That said, many vets will still source and administer it. Our vet (not the initial emergency vet mentioned above) was very supportive.
Yes! GS/Mutian/etc. saved our kitty! she was diagnosed with FIP and the emergency vet said we should put her down. Took her home to say goodbye and did some more research. A few Facebook groups and about 12 hours and we had a couple of vials.
The injections are horrible to administer and use a super large needle that really hurts the cats but it's worth it. We transitioned to pills after about a month when we saw that the meds were clearly working and she wasn't on the brink of death.
She's still with us today. Absolutely destroying our furniture.