Rather than be offended, sometimes it's easier to assume you aren't the target recipient, and all that's requested is your presence (and maybe a token gift).
A lot of people add ridiculously expensive items to a registry because they can get 10% off when they purchase it themselves. They also don't add all the cheap items because they don't know or care what kind of swaddling blankets or measuring cups they get, it doens't mean they aren't still appreciated. Some people have friends and family who absolutely want to purchase the china, kitchen-aid, stroller, or crib. Some people enjoy giving money for honeymoons or starter homes. Also showers are usually hosted, so don't always assume everything is the idea of the guest of honor.
Finally, just because you don't feel super close to someone, that doesn't mean they don't like you. Maybe you get invited to a coworker's baby shower because they appreciate your acerbic wit or think you'd decorate the hell out of a onesie. You never know, so contribute or don't according to how close you feel, and try not to read into someone else's intentions. If you over contribute they may well feel uncomfortable about it too.