As best as I can answer, it's mainly a money issue. Discussing units and armies only encourages the hobby, and GW would want this. However, with points costs and rules, it can be fairly detrimental. If we allowed people to post points costs and such, it wouldn't take long to find out how much everything costs. You wouldn't have to buy the codex's, if people onlined told you all the rules. On some sites, some people have been as blatant as to actually ask for scanned codex pages, instead of just buying the books. Thats a basic run-down of it