Actually, they do. You just have to know where to look.

When you have selected a wheel to go on the picture of the car, click the "View Closeup" button and it will show the offset on the info page.

*edit* Oh, and when you pull up your car they will generally only show wheels that will fit. Sometimes they will give you something that's outside of the ideal offset range so you should check just in case. Also doesn't hurt to give em a call--their phone people are actually really well trained about sizing and if like one particular brand's offset ratings are different than normal for some reason or another.