The public meeting was on the process of negotiating the treaty. They were quite specifically not discussing the content of the treaty, making the secret treat

The public meeting was on the process of negotiating the treaty. They were quite specifically not discussing the content of the treaty, making the secret treaty still quite secret. The rules of the US Government building aren't relevant. The meetings about the secrecy are being held in Mexico. They nearly required the attending public to sign NDAs and surrender electronics, but last minute deliberation removed those restrictions. Which suggests the ejection of a live-blogger appear quite unfounded. The ACTA meetings themselves are being held at some other time, in an undisclosed location in Mexico.