I very much doubt that the free internet connection is physically linked to the terminal that they swipe your card through. No way would they want to allow open access direct into their reservations systems. Yes, I am sure it can be done securely, but there would be a cost for it, and what would be the benefit?
The system worked for some time without any security. Apparently someone complained - either valid users who had speed/access issues due to leechers, or the airport authority losing $$$ on leechers. The simple solution was to add a password to the wireless router. This is a free fix for NW, other than the few minutes it takes to log into the router and put up a sign for the authorized users, who are controlled by their physical ability to enter the club and see the sign.
To require authentication means costs for IT people to program the interface, probably some new hardware/wiring, maintenance, and so on. Then there is the cost of helping users who don't have a WP # pre-authorized (internationanl biz class, partner clubs, spouses/guests, etc.). This then means longer lines for members trying to get fights rebooked by the agents. I can't see a return on this investment for NW, or the WC members.
If an underground network of hackers and password sharing causes whatever the original problem was to resurface, then they might want to look at more security. Otherwise, I find simple is better.