My Wifes 2005 ZX4 SE was purchased used and I looked at the tires before buying. They were all about 50% worn - all evenly. Today (about 1 year later) I found the Drivers rear tire was cupped and there was inside wear on the passenger front tire. A very similar thing happened to a 2001 Focus LX that we had previously owned. I know all 4 tires were brand new when I bought that car.
IMHO, I think its the way we drive. We live in the western North Carolina mountains and we drive like its the road track in Nascar - as fast as possible with hard cornering all the time. I just installed a strut tower brace on my new ZX5 SES. My train of thought is that the uneven tire wear is from the flexing of the strut towers drastically changing the toe in during our hard cornering. The SES has both front and rear sway bars, and with the strut brace it now seems to be the tightest suspension I have driven on. If my new tires wear well, then I guess my theory would be right. I plan to order one for my wifes ZX4, but unfortunately the SE only has a front sway, so it might not help handling as much with the looser rear end.
Anyway, could your problem be from your driving habits? I know the Focus is a popular car for people who like to drive pretty aggressively.
2009 Fusion- Vapor Silver SEL SAP V6
2007 Focus- Aqua Blue ZX5 SES
Stupid warranty! Stupid emissions! Stupid gas prices!