Paper = papel / newspaper = jornal. I agree sign is also a suitable answer, but do you think that would sound natural/correct to you: 'what newspaper do you sign?' This question points a routine. I dont think many people spend their time signing newspapers. So, in such case i think subscribe works better. It will depend on the context which one to use, more in English than Portuguese, cause in Portuguese they're the same. If one says "qual jornal você assina?" we'd see that as "what newspaper do you subscribe to?"
I agree with you to an extent, but there are a lot of questions and statements in this program that don't sound "normal" anyway. In fact we usually have to answer as literal as we can in attempt to evade losing our lovely hearts lol. Its more dangerous to assume and arbitrarily assign the meaning "subscribe" to "assina" when the whole time we've learned it as "sign". then on top of that the "help" selection can be misleading.
but like I said, I understand where you're coming from boss!
The olden days are not even really that old. The rule that a preposition can't be final was made up to begin with, and had more to do with Latin than anyone speaking English. For one description of the history, see here. http://blog.oxforddictionaries.com/2011/11/grammar-myths-prepositions/