Why is DHS frightened of Bitcoin? Because they, rightfully so, want to protect the American people from terrorist attacks.
DHS exists, first and foremost, to protect the private banking interests that have a monopoly on the issuing of currency and credit via the Federal Reserve. "Terrorism" is always the excuse used by political and money powers when they're defending their turf or looking to expand their control over a population. The largest terrorist organization on the planet, the USG, protecting anyone from terrorism is a lesson in cognitive dissonance. In political matters, always follow the money instead of the promise of being kept safe.
Bitcoin is a threat to the reputation of the Federal Reserve and other central banks such as the ECB. The Fed, via its proxy DHS, can't come down on Bitcoin all at once. If it did, it would show Bitcoin to be the threat it actually is. It will start small like this and then slowly build. By the end of the year, I fully expect to see "Bitcoin funds terrorists in [COUNTRY]" stories popping up in the corporate media.