Simply stated: "Why fight it"?
Cops allow it because it is going to happen anyhow so why not assume some control over the event? That is why everyone is in a sectioned off area so it can be monitored easier.
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As for the post asking about University of Denver (DU), it's a great school, good academics and good leadership from top to bottom. It is expensive though (but so is CU-Boulder if you are from out of state). The proximity of DU to Denver makes it a better choice than CU-Boulder. With DU, you can take public transportation to get to and from downtown quite easily and quickly. Plus, unlimited fare on the lightrail is included into your tuition (like $40 charge each semester). If you're in Boulder, you are a little isolated and the drive about 20-35 minutes to get to Denver....but Boulder offers modern conveniences too so you don't have to leave Boulder if you don't have to....it's just the nightlife in Boulder starts to get repetitive over the course of four years...
Personally, if I had to do it over again, I probably would choose CU-Boulder over DU, but hindsight is 20/20 and grass is greener on the other side...