Answer: It is easy to understand your rationalization that you are not making this public area any filthier than it was before you picked up the flyer. But let us consider at least two reasons for your taking the trouble to place that paper in the nearest trashcan.
First of all, once you have picked up that flyer to study its contents it is your property and you have no more moral right to litter the sidewalk with it than you do with any of your other possessions.
Secondly, the onlooker who saw you throw it away but didn't see you pick it up in the first place assumes that you are just an inconsiderate litterbug and is encouraged to follow your bad example.
By the way, don't worry about the loss suffered by the advertiser since he had no right to promote his cause by littering at the expense of the public.