Same reason that Bruce Wayne isn't afraid of falling off the side of a building. Something happens to you enough times, you kind of get used to it. Cheetah gets her tail hacked off on a regular basis, it's kind of a running theme with her, really, the tail-based defeat. At least twice, it's been the tiara itself that does it. If I were Minerva, I'd be hanging there going "my tail? Really? Really, Princess? That's so tired. Lop a toe off or something, just for variety's sake."