You clearly have a good grasp on brand marketing.
Why would you promote a brand, USPS that essentially has no competition and everyone is going to use anyway? That makes perfect sense.
Radioshack are raising their brand equity in the marketplace. The cost of sponsorship on a professional cycling team is about nil in comparison to lets say Nascar, or an ad on the Superbowl. Brand marketing generally doesn't get equated to an ROI because you can't attribute particular sales to people seeing Lance in a RadioShack Jersey.