As you said there is a limit to the amount of shoe throwing people , but if you go trough a main road that many travel by, you need a very small precentage of "shoe throwing" travelers .
Another point is that shoes do last for a long time , some of the shoes you see are probably pretty old , so they might be laying at the side of the road for a long time .
Also you should consider the fact that once you look for an object that is "misplaced" you will notice it more often, have you ever wondered how many pants, shirts , toys etc. end up at the side of the road ?
And last ( which I think explanis many starnge phenomenans ) "monkey see monkey do" once there are two shoes someone will find it funny to throw another shoe, after a month or so it becomes a trend !
