I also don't really understand the parasite thing.
They live with parasites in the wild, say someone with no other snakes found an injured ratsnake, nursed it back to health, but decided to keep it as a pet because it was eating well and was tame. People say they shouldn't keep it because of parasites..well, if it was doing fine with those parasites in the wild, why couldn't it be fine with those parasites in captivity?
I understand the mixing parasites with different species worry, kind of, but is that the only reason?