“We screwed up,” a top Capitol Police official said, speaking on condition of anonymity. He said Sheehan didn't violate any rules or laws.
By definition, if a reporter claims to have an "anonymous source," they don't or are lying about what was said. They made the comment up to support their particular spin on the event.
She did violate rules of the House, which I understand are enacted under procedures that make them have the effect of law. The rule forbids any disruptive behavior in the gallery of the House, which would include prominent protest signs or slogans on t-shirts (things like small lapel pins are OK). As noted they also ejected a congressman's wife for wearing a "support the troops" t-shirt (though she wasn't arrested) and, during the Clinton impeachment a person was ejected for wearing a t-shirt critical of ol Billy Jeff himself.
It is possible that the arrest was mistaken (though I bet justified by things she did and said), but clearly ejecting her from the gallery was lawful and according to House rules.
Maybe.
I'm thinking that the Congress critters put pressure on the Capitol police; you know how special they think they are.