Halliburton is still incorporated in the U.S., and Baker Hughes is headquartered in Houston so the merger will fall under purview of U.S. antitrust anyway

"The merger is widely expected to raise anti-trust concerns" http://www.reuters.com/article/2014/11/17/us-bakerhughes-off...

The US DOJ can still target, basically, any merger with global significance - especially if said companies do any business in the US.

You see the same thing when large American companies merge, they often have to get approval from the EU and increasingly Asian anti-trust authorities.