Oh, it's mostly through peer pressure.
Offices and other work places set it up so those that do not do the United Way thing stick out like sore thumbs, and sheeple seem to assume that if someone does not give to the United Way, they do not give at all, ("how can you be so callous"?)
Our office gives out "special" United Way T-shirts w/ company logo, then they have a "wear you t-shirt day", so those without them REALLY look like scrooges.
As far as I'm concerned, that's THIER problem, but most others' do not like the negative office publicity. How sad...