I believe it's because you can get in trouble for selling snakes if you aren't a petstore or a business of some kind. A way around it is to say re-homing fee.
if that's true I can see why people do it. But at the end of the day isn't it the same thing? Giving a animal in return for money. I don't think because you call it something else, that doesn't make it true. "no I wasn't selling this crack, I was rehoming it" lol :p