That's crazy. In the 2 decades I've lived in Portland, I've never seen more than the top inch or two freeze, and that's typically only for a few days to a week when a real cold snap sets in. Then it starts raining again and quickly thaws out. I can't imagine it has been cold enough for long enough to freeze down to 18" since the last ice age.
The problem is you can't have your buildings destroyed every 30 years with freak weather. So the frost line for building is set at X00 year depth, and then a little extra.