I think Boston.com defines what can and what can't be advertised on their web page. That's why no one sees x-rated ads or Ann Coulter there regardless of their browsing habits.
Sig Academy can be classified as training or school, boston area. Not just gun. So if they want schools to show up, they choose schools from the ad company.
Boston.com is not receiving ad revenue from SigAcademy. They get revenue from MediaPlex in the form of ad displays, and click throughs. You view it and it racks up a credit to boston.com.
Tempered by an article on the same page by James Alan Fox entitled 'Site shamefully sells rifle umbrella'. I wish that kind of crap was excluded based on my browsing history.