Hi Sasha, interesting question, so I did some digging and found various possible reasons why people do that in the US:
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.
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.
So theres many reasons why they sling shoes up in trees or wires. Hope this helped :)