Jetblue's password requirements specify that, among other stringent requirements:


  Cannot contain a Q or Z


I can't fathom a logical reason for this, unless it were say, extremely common for the left side of keyboards to break, but then you wouldn't allow 'A' either :)

What would be the reason for this security requirement?

It's a leftover from the time when keypads didn't have the letters Q and Z. Security-wise, there's no reason. It's just because of old systems.

To clarify:

You used to be able to enter your password over the phone. Some phones didn't have the letters Q or Z, like the one on the picture below.


  
  Image courtesy: Bill Bradford on flickr.com


Because of this, passwords including these characters were disallowed. They haven't changed this requirement for whatever reason: Legacy systems, poor documentation, or they just don't care. 
