Besides the obvious, which is they don't want anyone in there for their own safety, how do you traverse slippery slopes in the winter?  I have experienced great success using a device called Kahtoola's Micro Spikes. They worked so well, I bought a pair for my wife, my kids and their spouses, and even friends. They fit on your boot nicely, and bring traction to your feet using 1/4" V-spikes. 

I've used these extensively for the last several winters, and they are worth every penny.  My wife and I have hiked in so many places that simply were not accessible any other way. I've had more than a few times wearing them was my only way out.