"An ethnic group or ethnicity is a grouping of humans based on people who identify with each other on the basis of shared attributes that distinguish them from other groups such as a common set of traditions, ancestry, language, history, society, culture, nation, religion, or social treatment within their residing area."
In other words, a nationality IS an ethnic group, but so are other things. Not only that, but a person can belong to multiple ethnic groups. And while ethnicity can be based on your DNA, it certainly doesn't have to be.
So surely this would be ethnic groups. In some cases, ethnic groups have been largely combined in the American context (hence "Indian" and "Filipino" while the Hmong/Laotian distinction is preserved). But that WOULD mean accepting "Bengali."
As I've already said, the Census can't take everything into account and relies a lot on self-identification. Organizing things by nationality makes more sense from an official perspective because it's more concrete and less complicated than identifying the thousands of cultural/linguistic ethnic groups across America. If you have complaints with this system, take it up with the Census Bureau, not me.