Yes, this is a very fair question to ask. Of course, on the one hand, I could just say "this was fair game". The "point" of choosing to play on this server, is that it is THE no-rules server. The people who stick around more than a few months are the ones who accept that anything will eventually be griefed. Sometimes people call this the "sandcastle mentality", which means that a player accepts the destructive nature of 2b2t, and learns to live with it. Its name comes from building sandcastles on beaches: no one builds a sandcastle believing or expecting that it will last forever. The waves will always wash it away eventually, or someone else will break it. The only thing standing between your base and "the waves" is that no one knows where it is.

One way to think about it, is that in "normal" Minecraft servers, it's you versus everyone, but in an anarchy server that allows hacks, it's you versus the server software. There have been many hacks to locate other players, like following the trails that they leave in the world, that you can only see if you inspect some specific bits in chunk packets from the server. Perhaps they posted a screenshot that shows bedrock, which you can brute force their location from in a few hours/days on a GPU with OpenCL that simulates Minecraft terrain generation. Nocom was similarly "fair game", in that it only used the Minecraft protocol, and stayed within the "bounds" of talking to 2b2t with Minecraft packets and extracting information from that.

All that being said, it's still plainly the case that people put a ton of time into their bases, and no matter how much moral sleight of hand and cope you apply, it's still kicking over someone's sandcastle. When it comes to actually traveling to these locations and raiding them, our aim in that campaign was pretty much solely to amass a large stash of items/blocks for ourselves for the future. I would pass on bases by-default, only handing them along to be raided if there was bad faith construction (e.g. swastikas), or if most of the builders were known to be naughty. The main campaign was against standalone item stashes. These do take some time to create (by having exploiting past item duplication glitches), but they don't really have any creative expression in the same way that a base does. I have no logical argument for why I say that the same amount of hours invested in artistically making a build is more morally valuable than duplicating building blocks, it's just what feels about right. During the time when I was involved, we passed up hundreds of bases just because there was something actively being built there, and essentially only hit places that were just rows of chests placed on the ground in the world.