Failure to accept a rematch indicates your opponent feels they got lucky. It is rare people come on this site to play just one game then leave.
Some players behave in a way they would not in person. They may leave to give you a "slap in the face" or another rude gesture. If you only have one stradegy then this is usually the case.
IMO failure to rematch is cowardly unless explained with a short message on the chat board.
According to whose ethics?
Ethics of the chess community. Morals and values are more individual. Ethics are more universal.