Now, the ISP may have violated their privacy agreement, but privacy agreements usually contain verbiage that denies privacy if you are suspected of a crime, depending on the nature of information being divulged.
Now, if that information was somehow "unlisted at the user's request", like an unlisted phone number, then a warrant would be needed to obtain the information. I do not know of an ISP that provides "unlisted" Internet service.
I do not know of an ISP that provides a "listed" internet service, either. I can't find personal addresses from IPs, so it's not listed.
If the privacy agreement has a "suspected of a crime" loop-hole, a warrant would provide suitable, credible evidence of the suspicion. ISPs might be absolved by their clause, but law enforcement requires that judges validate that there is reasonable suspicion. This is standard procedure for modifying a citizen's privacy. An extra step, true, but not overly burdensome.