Scarbowl,
I'm not sure if you meant that I'm the one who doesn't go to the Woodlands or if you are the person who doesn't go to the Woodlands, but either way I think you might be missing an important piece of information.
For SAS families (and lots of Americans), the Woodlands refers to really two distinct areas: the broader area which encompasses the north of the island bordered by Sembawang, Bukit Panjang, Mandai, Sungei Kadut, etc. and the area immediately adjacent to the school which is restricted to the 'tree' streets: Woodgrove, Pinewood, Cedarwood, etc... If you live in this smaller area, the concentration of Americans is much, much higher than any other place in Singapore. It was around 94% a few years ago, not sure what it is now. I think this is the specific area the OP was speaking about since they mentioned not only SAS, but the Woodlands and houses as well.
So when people talk about trick or treating in the Woodlands, they are speaking about the smaller area next to SAS, not up Woodlands Avenue 12.