Immigration and Customs Enforcement is a DHS sub-agency. Organized copyright infringement like DVD and fake-product piracy, as well as contraband IP like child porn, has usually been their responsibility because a lot of it was done overseas and imported. Patrolling the internet sort of fell to them by default, and not that far away from stuff like cybersecurity. They're not the only government agency interested in such things, of course, but DHS has one of the largest budget allocations in the federal government.