The reason it says that is the offset on those wheels is all screwed up. Those wheels will be way inside the fenders and IMO look like crap. In addition to that why buy aftermarket rims and not get some that have some decent width to them?Mine came with the 17's but i want to upgrade to those wheels i showed.
Check these out. You could run 9" wide ones out front and 10" out back. And the offsets will have them sticking out closer to the edge of the fenders which will look a LOT better:
Black Bullitt Mustang Wheels (2005-2009) - Free Shipping!
I would go with 18x10's at all 4 corners with 275mm wide tires all the way around myself.:bigthumbsup