The Bears used to let you carry "one last" beer from the parking lot where everyone tailgates to the door of the stadium. Recently, for whatever reason, they started cracking down, saying that they'd arrest anyone who brought a beer out of the lot towards the stadium. Lots of people disregard the sign and do it anyways, but most Bears fans are relatively law-abiding so they toss that beer before they leave the north end of the south lot.
To make things a bit easier they put a big dumpster right near the pedestrian exit. As people walk by they toss that last half-drunk beer into the bin.
In Chicago we have enterprising bums. A lot of bums (you can see 4 of them from the guy in the blue jacket on the left all the way to the guy with the hat on the right) stand by this dumpster and waylay you to give them the last half of your beer instead of throwing it away. Since this seems more efficient than "wasting" that last beer, lots of people do so and the bums end up with a myriad of half drunk beers of various (mostly US) vintages.
Good thing the crack security at Soldiers Field is all over this...