Both things make sense together - people care more about not making a mess were they live than were they don't and left-wingers also think about others rather than just themselves - "do to others what you would like them to do to you" - which in this case means don't leave your trash behind to dirty up the place were others live, whilst rightwingers are all about what's best for themselves and in a place were they don't live simply dropping their trash on the ground and leaving it to dirty up somebody else's streets is less hassle for them personally than carrying their trash to the nearest trash bin.