Purchasing and downloading a single game on steam/battle.net/psn etc can eat 15GB+ of quota per person. Add wow/ffxi and youtube into the mix and we were easily exceeding the top levels of the available quota each month in our house hold with four people with no problems and that was 2 years ago.
And that is all legitimate traffic. Please don't just assume that people with large quotas are automatically pirates who should be investigated, it is like preemptively handing out speeding tickets to people with cars that can go faster than 110 kmp/h
Lets not forget that the techniques used are at best imprecise, laser printers have been handed DMCA take down notices for downloading pirated movies..... http://www.scribd.com/doc/28686586/Why-My-Printer-Received-a-DMCA-Takedown-Notice