has good reason to question why his edit was reverted.  His edit to the infobox seems quite logical as not everyone who reads Wikipedia resides in the United States and may have knowledge that Chicago and Illinois are linked.  Clicking the Chicago link does not show you Illinois you must look for it and vice versa for the latter. WP:OVERLINK not only clarifies this but also supports its use with the following line of text: Generally, a link should appear only once in an article, but if helpful for readers, links may be repeated in infoboxes, tables, image captions, footnotes, and at the first occurrence after the lead.   
 As for abbreviating to U.S. most everyone abbreviates that around the world, same as U.K. and in the past with U.S.S.R. so I see no issue there.