Craigly, you hit the nail right on the head. She constantly refuses to answer what her songs are about because she clearly doesn't write them. Much like they did with Avril Lavigne, what happens with teen singers is they get other people to write their songs and then credit the singer as a "co writer".
Taylor was asked in an interview to explain her song "You". Her response was literally "Uh um...you're the first person to ask me about that songs...um I dunno....it's hard because I don't have an answer prepared...uh..it's a sad song....". That was literally all she could say. She became flustered and didn't know what to say when asked about her song's meaning.
Singers often refuse to speak about their personal lives, but to "refuse to speak about what the song means" really translates to "I don't know what it means, because I didn't write it".