although your probably right about the reason they offer a rematch. they haven't caused any damage just decline and move on. if they hurl abuse at you for declining or say 'your a pussy if you decline' when offering a rematch then i'd recommend reporting them otherwise no issue.
Challenging a foe who lost
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I think people who challenge an opponent they just beat(especially if they beat that opponent easily) is a sign of a huge disrespect. It's like saying: "Cool, give me more points". I have encountered this many times and they must be terrible persons...
It's the opponent who should issue a challenge and only then they can accept it.