The answer may not be pleasing, but it's true: because "paid subscribers" represent a valuable advertising demographic/segment. They are nearly certain you are human traffic, and you have enough disposable income that you pay for news.
You can pretty much make this argument for everything - Ferraris should be plastered by LCDs with ads then. Business class airplanes filled with blinking video ads. Entrances to highend butiques crowded with people pushing ad leaflets into rich peoples hands.... Why is it acceptable on the web?
Not a Ferrari, but my mother got a discount on her Toyota Yaris by letting the car dealer put a decal ad on it for 1 or 2 years. (Back in 2002 or so.)