I've run into the "canon lawyering" problem in other rpg games, such as Marvel Superheros, DC Heros, various Star Wars rpgs, etc ...
Typically they're properties which have a lot of previous stuff written for it, such as background splatbooks, novels, comic books, movies, television shows, etc ...
You know, I think this is an interesting point to me, though probably for reasons you were not intending.
Canon lawyering is indeed quite common, but it is particularly common for things that have "lots of previous stuff", in other words they are adaptations of things that were not originally intended to be used as settings for many new original stories created by hundreds of different fans and DMs. For such settings, large quantities of canon are simply going to exist regardless of anyone's intentions, and are a necessary evil with regards to fan fiction and RPG campaigns.
However, D&D campaign settings are a different case. Ideally, and unlike the examples you listed, such settings are in fact designed with the idea that they will be used to create countless different and likely contradictory works of campaigns and fan fiction. Treating such a setting as like the DC universe, the Star Wars universe, or any other setting defined by a central ongoing story, may well be a mistake, because treating it in such a way may take away from the intended primary goal of that kind of setting: to be a resource for creating new campaigns in a fun and easy manner. In such a case, canon goes from being a necessary evil to simply being an evil.
To be perfectly honest, this is why I like Eberron and its approach to canon. No Eberron novel is canonical, no novel can change the setting, there is no ongoing story that progresses across editions, there are no central heroic characters, there are many concepts and secrets within the setting that are officially off-limits for the establishment of canonical descriptions or explanations (WotC will not create a canonical explanation for the cause of the Mourning, among other things), and many significant elements of the setting are left intentionally vague and self-contradictory (Kaius's motivations, the nature of the Lord of Blades, the goals of the Daughters of Sora Kell, etc). Eberron deliberately avoids the establishment of canon and leaves room for DMs to fill in the holes, which is something I consider to be a necessary step for making a setting that is flexible enough to let DMs to simultaneously use a setting as written and change things around or invent things to suit the needs of a campaign.
Of course, I should also admit that I really don't like many of the later 3E splatbooks for Eberron simply because they diminish the flexibility of the setting that was established in the original ECS. In fact, I have already tangled with a couple Eberron canon lawyers who were a bit too stuck up on one interpretation of some bit of canon from a later splatbook that probably wasn't all that well thought out in the first place. Ugh...
To get back to my point, I guess I will say that Forgotten Realms may be connected to so many complaints about canon lawyering and such simply because it modeled itself on things like Star Wars continuity or the DC universe more than it should have, when a different approach works better for something that needs to be as open and flexible as a D&D campaign setting.
