Most Helpful Guy
In regards to your initial question: As a courtesy, just be honest and forthcoming about your reason (s) for ending the sexual relationship with him.
"Why do some guys opt to stay friends by no sex? Or, why they try to just be nice to keep you from hating them?" Likely, to keep the door open for a future sexual relationship.
Most Helpful Girl
Who knows. Maybe that's just the type of person they are or they don't want terrible relationship karma.