Since when does Homeland Security police copyright infringement?

Ensuring the security of corporate profits.
I think in the Cyrillic alphabet, DHS is written KGB.

Did you ever really think that a massive organization like DHS was put in place to monitor the activities of a few dozen suspected extremists?

It was "a few dozen" before the Cheney-Rove Recruiting Program.

You know, just about every album you can think of and quite a few movies are posted on You Tube in their entirety. Where's DHS on this? Or are they too busy chasing Edward Snowden and raiding movie theatres?

The only reason those posts are on YouTube is because the copyright owners have not sent cease and desist orders to YouTube. There are a lot of things that used to be on YouTube that are no longer there.

Keep in mind: if an agency is part of the U.S. government, it is part of the corporate oligarchy that actually runs this pitiful country.