Following the 911 attacks it is impossible to hijack an airliner with a 4in blade and the enormous resources spent preventing this possibility would be much better spent on real threats.
I totally agree with Tim S
Although unlikey it is all about risk management / avoidance why risk it.
We all know this lady was probably harmless and just practicing her faith but some people out there play on that and wouldn't hesitate to start attacking passengers in order for the Capt to divert to somewhere of their choosing.
Just because the flt deck are safe behind the door does not solve the problem.
Airports and Airlines have a duty to all the passenger to ensure they are also safe from harm