I get what you're saying.
However, I'm guessing that they were the more "outdoorsy" types. I personally hate being cooped up all winter, so the nice weather that day probably seemed like a welcome break. According to all the interviews they were very active girls interested in things such as crafts, sports, knitting hats for NICU babies, hikes, camping etc...They probably were just wanting to get out and take some artistic pictures of the park since the weather was nice. In fact, if I'm not mistaken that's what Libby's grandfather said in a very recent interview.
As for why they weren't scared, I think it was probably a place they felt so happy and comfortable with that it almost felt as safe to them as playing in their backyard. One thing is for certain though...I wish they hadn't gone.
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