One being the parents are so cheap to be had. You don't have to worry about wasting a year or not selling the babies or anything like that. They are cheap! I would be more reluctant to breed my albino RT boa with another species than I would be with my snow cornsnake LOL.
I am not really missing out on much if I dont have a clutch of pure snow corns, but I would be missing out big time if I had some mutted up clutch of boas instead of pure true albino RT's. (just example using somewhat middle end snake)
Another reason is alot of people justify corn crosses with certain things as being natural intergrades so it makes them feel better about it I guess. I don't feel one way or another on the issue myself, but because I myself prefer the looks of the pure snakes I will be keeping all mine pure.
I would also think its kinda like "throw away pets" people don't get in much trouble for abusing goldfish, because they are "worthless" I am not saying hybrid people think these snakes are anything but amazing creatures, BUT corns and kings are like the goldfish of the snake world somewhat, and easy to "mess" up or breed to another species etc.
That's my guess. Like I mentioned, I personally don't care either way aside from the fact I prefer the look of pure species.