It was a marketing thing many years ago, IIRC. Alton Brown did an episode of Good Eats on Sweet Potatos that delved into the history of this "deception." I think it was that the African slaves who came to the South and saw sweet potatos called them yams from remembering the similar looking tubers they were used to. It stuck. There's probably some deep sociological implications there.
Sweet potatos (or is it potatoes, as Candice Bergen would point out) are much closer to regular spuds than to yams. See this thread on the blog Homesick Texan for more. Isn't it sad? I'm still working on the thread from 11/25 and I took the time to look this up. My life, at this time of year, is really lousy. You don't want to know how much work I'm doing on my "vacation" today and tomorrow. I promise, I will post the awards from 11/25 by tomorrow morning, at the latest. I'm assuming that jmyrlefuller is working on the awards for the posts from the Sunday shows broadcast on 12/2, but I haven't heard back from him to confirm that. For all I know he may have already posted them. It's been that sort of week.
I am what I am.