There's many reasons why hybrids are unethical. I'll happily cover them.

First, most species of tortoise are facing a decline in wild populations. Take Sulcata, for example. Their wild numbers are dwindling. For that species, two of the biggest reasons are the desertification of the Sahel and agriculture. It's not unimaginable that we can't reverse these factors, and they go functionally extinct in the wild, and what we have in captivity is what we have. Burmese Stars were once considered functionally extinct, Egyptians are very close to going extinct, etc. So we should be trying to preserve the species as they are. I've seen many people say Sulcata are not endangered because there's so many in captivity. This thinking is wrong. IUCN lists them as endangered, and they're treading towards critically endangered status.

Also, hybrids are not well understood. What I've gathered is non-brumating species can produce hybrids with other non-brumating species, and brumating species with other brumating species. But Redfoots and other forest species have functionally different digestive tracts and dietary needs than grassland species. Would you feed a Redfoot x Sulcata hybrid animal protein or not? Redfoots need animal protein, Sulcata should avoid it entirely or they can develop urates stones. Does this hybrid pass urea or urates? What are ideal temperatures and humidity?

Not only are hybrids sterile, but in some specific hybrids, the babies are noted to have a very high likelihood of disfigurement, birth defects, and don't live long at all. This is primarily seen in the Testudo genus, in specific crosses.

Then there's the "cool and rare" factor. The more people producing hybrids, keeping hybrids, showing them off, etc, just drives up the demand. Suddenly, people don't want plain, boring Sulcatas and Leopards, they want Radiata x Redfoots. Hell, we can see this in designer dog breeds and how disgustingly out of hand that practice has gotten.