This doesn't make grazing land unsuitable for cropping. As an example, a lot of dairy farming is done on fertile volcanic soil that would also be suitable for root vegetables.
"Cropland ... Includes: All land from which crops were harvested or hay was cut. All land in orchards, citrus groves, vineyards and nursery and greenhouse crops. Land in rotational pasture and grazing land that could have been used for crops without additional improvements. Land used for cover crops, legumes, and soil improvement grasses, but not harvested and not pastured. Land on which crops failed. Land in cultivated summer fallow."
By definition cropland is where you could grow crops, the rest is grassland.
I think "grazing land that could have been used for crops without additional improvements" (emphasis mine) doesn't include e.g. established pasture which could be easily but not trivially used as cropland, right? I would have assumed it was a catch-all for dual purpose crops etc.