[jk700]

Privacy became a reason when ISPs started providing internet access over Ethernet. As hubs and unmanaged switches are vulnerable to sniffing, ISPs had to either give everyone separate VLANs or use managed switches with protections from sniffing.

[paleotrope]

Like in a datacenter or carrier hotel? You make it sound like carrier ethernet which I don't believe ever had anything like this as a problem.

[jk700]

A typical FTTB ISP providing services to people living in apartment buildings isn't doing a "carrier ethernet", I guess it's more like a datacenter. It just has a media converter connected to a switch or these days a switch with an SFP port per building and from there it's all regular copper and RJ45. In the end of 90s and early 2000s ISPs haven't yet figured out how to do it properly, how to isolate and authenticate customers, at the time there were plenty of hubs and dumb switches with plenty of sniffing going on, plenty of vulnerabilities in switches, a single customer was able to cause problems for the whole network.