I think the description of a rib roast as a prime rib is acceptable, although not entirely accurate. Similar to various store labels as "managers choice" "Kansas City Prime", or Angus Prime, all trade names. What is not acceptable to the USDA is the labeling of a cut of beef as USDA choice or prime if it has not been graded.
Personally, when I buy prime, I buy USDA prime. It is readily available if you want to pay the price. Costco carries it. Yesterday at Costco, whole boneless ribs were $10.99, steaks $14.99. Whole Choice was around six bucks.
Grading is a judgment call, and based on the appearance of only one steak and the overall configuration of the animal. It is sometimes possible to purchase a better cut of beef labeled USDA Choice than USDA Prime at the same bin. It pays to know your meat. It is particularly important if you are buying cut up meat, since one end of the rib is next to the chuck, and the other is next to the strip. In strips it's the rib and the round. In general, the closer to the center of the animal, the better the steak.