Hmmmm. Will I be able to read the articles that usually require a subscription to BPL if I buy the app? If not, what added value, other than being able to use my iPhone, is the app offering? If yes, then why would I want to subscribe if I can get the same thing from a $2.99 app?
Personally, I'd see the app as a way to expand, for lack of a better term, BPL market penetration. You're unlikely to do that if you charge for it, as BPL isn't some 'name' that people think they just have to have. The app is a gateway to more subscribers, that kind of thing. Charging for it limits that potential. IMHO.
EDIT: Just re-read your initial post. I get the last six weeks of articles if I buy the app. Still don't see enough of an added value to actually pay for it, and still think that charging for it limits its appeal to expanding the BPL universe.