There is really no good answer and it is probably not a drug dealer sign ( it is really commonly known urban legend - but not an efficient sigh, mainly because the police is aware of it as well ) in any case I have found that odd to and I have talked to an urban legend entousiast which told me of other possible reasons like :
It's the work of gangs marking the boundaries of their territory.
Bullies take them off defenceless kids, then sling them up out of reach as the ultimate taunt.
Gang members create an informal memorial at the spot where a friend lost his life.
The shoes increase wire visibility for low-flying aircraft.
Overly puffed-up boys who have just lost their virginity or otherwise passed a sexual milestone look to signal the event to others.
Graduating seniors mark this transition in their lives by leaving something of themselves behind; namely, their shoes.
Kids do it just because it's fun. And besides, what else are you going to do with a worn-out pair of sneakers other than tie the laces together and toss them high?
most of them are unlikely, but I like the last one.