I believe I read that the NAB had assisted in writing the bill. I think they are trying to distinguish between "normal" AM IBOC HD radio which has an analog and a digital component and true all-digital AM HD which has no analog component.Swell, so congress has crafted this legislation in accordance with its usual level of comprehension of contemporary technological and technical subject matter.
It is interesting that we still use the "IBOC" (In Band On Carrier) designation which was put forth as a selling point against DRM, which requires a separate frequency, transmitter and facility for digital. Nobody is proposing DRM anywhere in the Western Hemisphere so there is no need to distinguish between the two systems in the U.S.