I think you are essentially right about DoubleClick, but the concept of 'spyware' overstates what I think they were doing.
In the US your mailing address allows you to be associated with your census data by census track. We can then know if you are in an urban or rural area and an approximation of your income. Telephone markets can know essentially the same things by using your area code and exchange.
Marketers can then aim their marketing to the people mostlikely to buy a product.
This use of census data could be cast as 'spy' data if you wanted to inflame people.
The Internet does not have an easy way to determine the demographics of people on the Internet. IP addresses can't be used this way.
So some bright marketer though of the idea of tracking where you went as a method of determining what interested you and what kind of businesses attracted your attention.
One side of the coin is called 'spying' and the other 'customer targeting.' In order for commercial enterprises to exist on the Internet, there has to be a way for an advertiser to reach his customer.
DoubleClick may have been caught in the spotlight but I doubt that they were doing anything that is common marketing practices in Direct Mail advertising.