Hi Myriam! It seems like abuse of the cultivar designation to me. If they aren't patented or registered with some kind of international naming authority, there is no official documentation that a cultivar exists.
I think "they" do it because they think people like or need a "fru fru" name for each plant. And it makes the collectors want one of each. "I can't see the difference but the tag says it's one I don't already have" kind of thing.
I prefer to not buy actual cultivars because they're so often disappointing, not as vigorous a the straight species, don't really look different enough to bother, or as long-lived, but that is (supposedly, if you believe all of the labels) all that is available at stores. If you believe the labels, even the "weeds" they sell have cultivar names, like St. Augustine & Bermuda grass, Vinca major, Callisia repens, Nandina, English ivy... Maybe giving them a cutesy name makes them sound less like weeds. UNLESS it's something that really does have a cultivar name, like a Begonia. Those rarely have the name included.
