In response to loganbeary at my Google+ post on this topic:

I think the paranoid theory about the rocks is that it was an intentional act of provocation/entrapment. Either by a private citizen...
http://crooksandliars.com/karoli/conservative-blogger-hands-out-bongs-ows-ho

... or by the government itself...
http://en.wikipedia.org/wiki/COINTELPRO

... and, of course, media's attempt to ingratiate itself with authorities. Luckily, media in Minneapolis is generally performing better than in other cities.