> Why is the DHS tracking down copyright infringers
When DHS was established, a bunch of existing government agencies were rolled into it. One of them was Immigration and Customs Enforcement (ICE), which is the federal agency with the most staff handling international IP law enforcement (traditionally, mostly counterfeiting). Copyright also falls under that category, so now DHS ends up being the one that investigates copyright infringement at the federal level.
I'd guess the GP's point was why are the DHS getting involved in what is primarily a civil action (tort) between a rights holder and an alleged infringer.
When DHS was established, a bunch of existing government agencies were rolled into it. One of them was Immigration and Customs Enforcement (ICE), which is the federal agency with the most staff handling international IP law enforcement (traditionally, mostly counterfeiting). Copyright also falls under that category, so now DHS ends up being the one that investigates copyright infringement at the federal level.