Convention hall network management question

Hey folks, question for you. How does a convention hall prevent exhibitors from connecting routers to the show's internal network?

Some background, I'm at a show where I bought the lowest tier internet service, which is 1 ethernet line coming in to the booth. I typically plug this in to my router's WAN port, then run my LAN off of that with all external traffic getting NAT'ed. This time however my endpoints were not able to route out of the network, even though the router itself was showing good internet connectivity during ping tests. After some troubleshooting I eventually learned that the show's ISP was offering a "router package" for about $4000 more than what we paid, and that my package did not support router functionality..

Long story short they were able to detect that I had connected a router to my line, and prevented outbound traffic from passing through it. I am wondering, how? From my perspective I would think they just see 1 MAC address and 1 IP (the router's), so how can they prevent traffic that originates beyond their reach (my LAN)? How could they determine that pings from the router were okay, but other traffic from the router (my LAN's outbound traffic) was not okay? My best guess is they analyze outbound packets (frames?) to look for evidence of NAT, but I am no expert.

To be clear I am just asking this out of curiousity, not going to exploit or anything like that (although $5k for 4 days of internet is insane).

Thanks in advance!