I plugged into ethernet (as wifi w/captive portal does not work for me). I think clearnet worked but I have no interest in that. Egress Tor traffic was blocked and so was VPN. I’m not interested in editing all my scripts and configs to use clearnet, so the library’s internet is useless to me (unless I bother to try a tor bridge).
I was packing my laptop and a librarian spotted me unplugging my ethernet cable and approached me with big wide open eyes and pannicked angry voice (as if to be addressing a child that did something naughty), and said “you can’t do that!”
I have a lot of reasons for favoring ethernet, like not carrying a mobile phone that can facilitate the SMS verify that the library’s captive portal imposes, not to mention I’m not eager to share my mobile number willy nilly. The reason I actually gave her was that that I run a free software based system and the wifi drivers or firmware are proprietary so my wifi card doesn’t work¹. She was also worried that I was stealing an ethernet cable and I had to explain that I carry an ethernet cable with me, which she struggled to believe for a moment. When I said it didn’t work, she was like “good, I’m not surprised”, or something like that.
¹ In reality, I have whatever proprietary garbage my wifi NIC needs, but have a principled objection to a service financed by public money forcing people to install and execute proprietary non-free software on their own hardware. But there’s little hope for getting through to a librarian in the situation at hand, whereby I might as well have been caught disassembling their PCs.
I would have expected a public library, run by the city to either use the existing Internet infrastructure from the city (e.g security already is handled) or be installed and maintained by some common city IT team.
Independent libraries sure can have a basic setup, but I'd still say one guy setting up the security outside of WiFi security would mean there's no reason to fear ethernet connections, as they would provide the same level of security to their network, and likely more to the user (assuming it's an insecure AP with portal).
In the case of the OP, I would find it far more likely that the actions of the staff member was more down to (understandable) ignorance of what they were doing and assuming connecting a wire means they're trying to do something nefarious, just because noone else is, and/or hacking in all the movies looks just like that.