47 U.S. Code § 551 (c)(1)
Except as provided in paragraph (2), a cable operator shall not disclose personally identifiable information concerning any subscriber without the prior written or electronic consent of the subscriber concerned
So this section of the law, though another might, does not support your statement that "that they could sell that (which is false)".
Your statement is too strong. It's not outlawed. There is a relatively easy avenue for ISPs to sell this type of data, and it's written into the law, and not some sort of weird loophole. It is voluntary that they have not pursued it on their part.