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The reason for the takeoff / landing rule is potential interfering with emergency evacuations. Same reason things can not be on the floor in the row at those times.
As for this case, the FA was perhaps being a bit overzealous (with a "personal rule") but if it could be a issue for someone boarding / getting to their seat (especially since the OP was in an aisle seat), it does at least have a potential reason for suggesting to wait a bit until airborne.