Why are shoes thrown over the power lines?

I've found a few possible answers on the web.  Here they are:

• It's a time-honored tradition to throw your sneakers over the power lines on the last day of school.
• Because someone felt compelled to try to sling them up there so that they would stay (sometimes they are old shoes that need to be discarded, sometimes they are someone else's shoes, and the boys involved are playing a prank on that person.)
• Drug dealers use them as a sign that to let their customers know that drugs are sold in that area.
• To serve as a reminder/warning of a murder that occurred nearby.
• Because teenage boys who've just "scored" for the first time (i.e., lost their virginity) use that sign to proclaim their conquest to the world.
• They are left there as breeding pots for the extraterrestrial black oil virus. The life forms hide in the shelter of the shoes and observe traffic, waiting for just the right moment to commence the invasion.

Sources:
http://urbanlegends.about.com/cs/factoids/a/sneakers.htm
http://www.straightdope.com/classics/a5_274.html
http://www.snopes.com/crime/gangs/sneakers.htm
http://breebop.toastmedia.com/archives/000599.html
http://www.fodors.com/forums/threadselect.jsp?fid=1&tid=1403250