It's stunning how bad many card issuing systems are (as noted in the post, AmEx et al). When I was in college all of the administrative buildings, student common areas as well as many of the student housing areas were controlled by magstripe. Meals were also kept track of by card. I knew from people losing their cards which continued working some places but not others there was a relationship in the issuing. I got a…
It could have been "good enough". Remember the whole "keeps an honest man honest" bit. Retooling their security system might cost a lot more than some spoofed meals, and we all know doors & tumbler locks are never impervious.
I had a similar experience at my university. I found easy unauthenticated sourcing of most of the data needed to clone the card of anybody by name. The issue number was the only thing to guess, but easy to bruteforce on something low-stakes like vending machines. The card was used for food, a debit-card-like system, automated door locks to semi-public buildings and on-campus housing.
With the permission and cooperation of the university security, I made a card of a high-level security guy (who could have been targeted using the public/semi-public org chart) and swiped into their datacenter where all the university data is hosted, along with that of some partners with sensitive data. Luckily the innermost parts need an RFID or something which I didn't have access to, but potentially I could have tailgated or social-engineered my way into that. They weren't interested in letting me research whether I could crack the RFID. :(
I was told my demo made a big splash, but IIRC I checked a year or two later and my source for the ID data was still wide open. There's having imperfect locks and then there's leaving all your keys out in public.