I love Sweet Potato fries. When given the choice between the 2 I almost always choose SP.
Now having said that I think choices are good. Why not serve both?? Heck add in the apple slices or grapes in too. What skin off there back is there to do that?
My guess is this change is part of the Disney effort to go healthy. When they changed the kids menus at WDW people (especially on message boards) went nuts. Most people are used to the changes now.
I tend to go with the "when on vacation eat what you want" philosophy. The thing is many people (myself included) want to eat healthy and it can be harder to find those choices. So why not serve both? My guess is they say you can get regular fries at all the other places in the park, so go do that.
Oh and SP fries are much better with a little honey mustard dipping sauce. They really should serve that with it. It is pretty low fat and low in calories too.