Could be that no one was asking about the specifics concerning digging or removing items? When asked to leave, they just pick a different time and day of the week to return, hopefully when nobody was around. Nobody there, nobody cares, and there wasn't the words 'metal detecting' to be found in the rules, no signs clearly stating. Sure, it's public land, but the public also hired some one to oversee those sites, maintain them, keep them clean and safe, enforce the rules as needed. The rules concerning digging, removing items already existed, and apparently being ignored by a specific group, who found some really nice, old stuff, shared with their fellow hobbyists, who also want some of that good stuff too. If there is good stuff being found, there must be more... So, there was some grey area, and the person in charge could have allowed hunting, if it was done with care, a little respect, but these guys never ask, and are careful never to be seen, so no discussion was possible. The man has got a job to do, he's responsible for that particular site, deserves a little respect, I'm sure there are other problems he has to deal with.
Sure, a big part of research, is looking things up, a lot of reading, but when you get into those grey areas, not clearly defined, you need to consult an expert or authority on the subject, to gain clarity. That authority, would be the person most directly responsible for the site you wish to hunt.
Laws are frequently model or copied from other areas, even other countries, it's no secret, nothing new or unique, just a very common way of doing business. Takes a lot of research, trial & error, to create something from scratch, we all adapt work already done, to fit our specific needs. We make use of things already known, build or improve, it's how we keep moving forward. If we want to keep our rights, we need to communicate, show we have an interest, respect the wishes of others. Keeping silent, implies we just don't care about the rules, will do whatever we want, regardless. The rules get stricter, more specific, with more compelling consequences, until we do care, and we do find our voices. Sure, we are going to get told 'No', but we can build on every time we get a 'yes', and show we can be responsible, show some respect. Rules can be changed, but you have to work at it, earn it.