Some Americans might call sweet potatoes “yams”, but some use that word to mean yams.
The grocery stores I’ve seen that have both have them right next to each other, at the same price. If customers still get confused, well, at least it’s not the store confusing them.
What’s confusing is that at many stores I will see garnet or jewel yams labeled as “yams” and the rounder type of sweet potato labeled as “sweet potato.” All are varieties of sweet potato. If it has a fairly smooth skin, it’s not a “true” yam. The one on the left here is a true yam. It is not commonly seen in American groceries but, like I said, you do see it occasionally (as in my local grocery.) The one on the right is the garnet yam, and what many people in the US call a “yam.” There are many types of sweet potato, of which the garnet yam is one. It has an orange flesh. Regular sweet potato usually has white flesh, but orange-flashed varieties are also often sold simply labeled as “sweet potato.” And true yams are usually white/yellowish-white fleshed on the interior.
There actually is a clear, distinct difference between a sweet potato and what’s properly called a yam.
There is not a clear, distinct difference between which sweet potatoes are called yams in the USA and which are not. Or rather, there are quite a few people who will give me a clear difference; but they disagree with each other.
I’ve also seen a number of explanations of the difference between cantaloupe and muskmelon that disagree with each other; including explanations that say one is a type of the other.
At market I label them sweet potatoes and melons, and label them additionally by variety. Anyone who asks gets told that the whole thing is confusing; though the is-it-a-yam people get a somewhat longer speech than the is-it-a-cantaloupe people. They also get a short description of the variety and an offer of their money back if they’re not happy with the purchase. Almost nobody demands their money back.
ETA: sometimes, to further confuse matters, I grow a variety with “yam” in the name. It may or may not be what a specific purchaser expects from a “yam”. “Japanese Yam”, for instance, is a purple-skinned sweet potato with pale flesh.
