I have noticed that Keepsake offers 44" "backing" fabrics that have been precut for their quilt kits, and you can buy them by the yard.
http://www.keepsakequilting.com/prod...ic-Backing.htm
When you search for backing on that site, you get lots of fabrics that don't mention the width, as well as lots that do. I don't know if this is where you ordered from, but you're right - let the buyer beware. I would fully expect that fabric advertised as "backing" would be at least 90" wide, but apparently not.