US worried about Iran attacking US forces in Iraq?

Politico:

As negotiations on a possible nuclear deal approach a March 31 deadline, U.S. officials are increasingly alarmed about Iran’s expanding military presence in Iraq — and the threat it may pose to American soldiers in the country.

Two scenarios are of particular concern, officials say. One is that a collapse of the nuclear talks could escalate tensions between Iran and the U.S., emboldening Iranian hard-liners and potentially leading to attacks on Americans in Iraq.

The other is that increased U.S. efforts to oust Syrian president Bashar Assad, a close ally of Tehran, could provoke retaliation from Iran. White House officials who oppose greater involvement in Syria’s civil war often cite concern for the safety of Americans in Iraq as a reason for caution, sources said.

In either case, U.S. officials fear, Iran could direct the Iraqi Shiite militias under its control to attack U.S. troops aiding the fight against the Islamic State in Iraq and the Levant.
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This sounds weak to me.  If this is a legitimate fear, it is because Obama and Kerry are not willing to tell Iran that the US will pound the crap out of them if they attack US forces.  That is what is known as a deterrent effect.