How do you think devices broadcasting radio signals are detected?
That policy is almost certainly a paraphrase of the actual policy, which I suspect based upon experiece is actually:
We don't want people providing unsecured wireless hot-spots to the world. We give IP connectivity to you, the person with whom we have a contractual relationship, not to you and anyone else who just happens along. We don't want freeloaders to be able to piggyback off our service to you. We don't want random people with machines full of malwares to gain access to the organization's internal network. We don't want other radio signals interfering with our radio signals.
See the Rose-Hulman Institute of Technology's Wireless Router Policy for an example of one such actual policy.
It's fairly easy to detect wireless routers, simply by doing what
RedGrittyBrick alludes to. Looking at what's on the wired side of things, at mac addresses, IP addresses, and whatnot, is doing things the hard way. The offending devices are broadcasting a signal. (If they aren't broadcasting, they aren't part of the problem being tackled, of course.) So one just wanders around with equipment capable of detecting and receiving that signal. An ordinary laptop personal computer with a radio NIC would suffice.