This is an example of "price discrimination" which is explained in almost every introductory microeconomics textbook. It occurs because the demand for BDL-ORD-SFO travel is more sensitive to price than that for BDL-ORD travel. This could be because, as has been suggested above, there more alternatives on the former route or because the former route is more leisure travel while the latter is more business travel. The seller must be able to prevent buying in the lowered priced market and selling in the higher priced market hence the prohibition on hidden city ticketing. Many apparent quirks in airline pricing behavior can be explained as price discrimination.