When FM played PBA a few years ago, Stevie referenced a close, long-time friend of theirs that lives in Lincoln. So there may be a genuine Nebraska reason for the hat. This was also just a few years removed from the '70/'71 titles, so I'm sure there were a lot of non-Nebraskans "representing" at that time.
Either way, very cool photo. Makes me like Mick even more and I was already a fan.