I grew up in NYC in the late seventies/eighties, and I remember seeing shoes on the wires all the time. At one point my dad who was born and raised in Brooklyn explained it to me.
Apparently when school got out for the summer, the routine was to take your gym shoes, which were usually beat by then, and fling them up onto the wires. Sort of a mini celebration. Probably akin to throwing your mortarboard after graduation.
Sort of...
When I was of gym class age, (early eighties) this must've been on its way out, 'cause I dont remember me or any of my friends doing it.
By that point we didnt really have specific "gym shoes" though. Everybody was pretty much wearing sneakers all the time. (the post Converse All-Star age of Reebok/Nike)
Anyway, that was my take on the phenomenon.