Yep, pure greed.
Piracy cases, in the US, are handled in civil court. All the plaintiff has to do is prove that it was more likely that you engaged in piracy than you did not. In other words there was a 51% likelyhood that you were sharing copyrighted files. ISP make mistakes in their IP databases all the time and they often fall out of sync. If an ISP stands to profit they may even alter the database to strenghten their case.
Even if you are accused of copyright infringement (and are innocent), either you pay up for "amnesty" or you pay a lawyer thousands to conduct the court arraignment, obtain evidence, have your computer analyzed by an expert, and hold preliminary court hearings. Your legal fees can soar to tens of thousands if the case goes to court.