Crowdsurfing only works well if it is clearly telegraphed by the surfer, and willingly accepted by the crowd - think like an enthusiastic trust fall. Given that she's 5'6, I can't see how she would have been landed on full force unless there was basically no warning, or the rest of the crowd around her totally bailed (in which case no-one would jump)
I'd argue that a "no crowdsurfing" sign would, for a punk band, be like a red rag to a bull... Given that she'd attended lots of their shows in the past, I think she'd have had a better gauge of how likely crowdsurfing was, sign or no.
I'm not blaming her at all - I meant that she'd have a good idea of whether or not the singer is a crowdsurfer, regardless of a venue sign being present or not. It's either the singer's fault for launching into the crowd with absolutely no warning, or the rest of the crowd for seeing the singer coming and scattering from around her.