The only thing data caps do is generate more money for the provider by charging heavy users for the excess data they use. Whether they actually use that new money to improve their towers is a completely different topic; they certainly haven't been improving capacity well enough for the past three years, when they already generate plenty of revenue. Why will it be different with caps in place?
Edit: I saw the same thing happen with Time Warner Cable in Rochester, NY. Just s/AT&T/TWC/ and s/towers/copper/ and it's the exact same situation.