As many people have pointed out, Hallowe'en at Microsoft is a very family-oriented affair. The parents love it because the kids are in a safe environment; the kids love it because instead of having to go from house to house, you just have to walk to the next office, and then there's a big party in the cafeteria afterwards.
Though it does lead to some misconceptions on the part of the younger children, the result of extrapolation from too few data points. I remember a colleague of mine who brought his young daughter to work for Hallowe'en. A conversation with "Cathy" some days later went like this:
"Do you know where your daddy works?"
"He works at Microsoft."
"Do you know what your daddy does at work?"
"He walks around and gets candy."