according to wikipedia.org:
Some say that shoes hanging from the wires advertise a local crack house where crack cocaine is used and sold (in which case the shoes are sometimes referred to as "Crack Tennies"). It can also relate to a place where Heroin is sold to symbolize the fact that once you take Heroin you can never 'leave': a reference to the addictive nature of the drug. Others claim that the shoes so thrown commemorate a gang-related murder, or the death of a gang member, or as a way of marking gang turf. A newsletter from the mayor of Los Angeles, California cites fears of many Los Angeles residents that "these shoes indicate sites at which drugs are sold or worse yet, gang turf,"
that's all bullshit though, invented by adults and nerds. kids know it's just fun to tie them shits together and let 'em rip.