It's called fear of the unknown. By 1977, clearly they weren't afraid that Dempsey was making the game unfair but what if somebody comes along later with some type of super balls built into the wood like a corked bat? With no rule to outlaw it, the league would find itself in a possible legal pickle. I don't consider his record tainted at all. I consider it miraculous.
One of the little-known stories about the kick is that the Saints had fired their head coach and promoted an interim. When the Billy Kilmer pass moved the ball to the Saints' 45, the coach asked "what line are we on?" and was told "the 45". He thought it was the Lions' 45 and sent Dempsey out to kick a 53-yarder. Only after it was too late to change course did he realize his mistake. He never thought Dempsey was capable of making it from that distance and Dempsey, for his part, was just doing as his coach instructed.