I get what your saying here, but what I don't get is why a blanket statement at the beginning of the MM stating that each campaign may have different monster lore can't work. There's no need to arbitrarily foist every monster onto a specific setting.
So, to your question: blanket statements like that aren't functional. You'd think they would be, but no, they actually aren't, in practice. 2e made a point of saying DMs make their worlds, but still people felt that the Blood War was foisted on them. 3e had Rule Zero and people still felt like wealth by level was gospel. 4e didn't have Fun Police but people still felt like succubi being devils was
the worst.
It doesn't do what it's meant to do. The people who make these criticisms aren't idiots, and they aren't "wrong," the games just aren't living up to their own message -- they aren't DESIGNED or PRESENTED to be played like they occasionally claim to be played as.
Also, the world assignment is not arbitrary. Every monster already has a specific setting that it's a part of. Typically in historical D&D, that's FR or Greyhawk (or the 2e omnisetting) and is left un-stated, but every monster has a specific context. There's no reason FR mariliths need to be defined by their Planescape lore, and no reason PS mariliths need to play by FR's rules, either.
Dasuul said:
Most people don't sit down, pick up the Monster Manual, and read it cover to cover, carefully assimilating every word.
...
In 25 years of gaming, I don't think I've ever read the opening pages of a Monster Manual in their entirety. Why would I? It's a reference book, not a novel! I bought it because I wanted to read about monsters and monster lore. The rest, I'll look up when I need it.
Apparently you're the kind of player that [MENTION=184]Agamon[/MENTION] thinks can't put on his pants.

That's totally a part of what I was talking about above with a paragraph in the opening being ineffective. If people don't read it, certainly they are going to miss the fact that this presented default cannot be assumed.
Second, you're contradicting yourself a bit here. Why would you read the MM? Because you want to read about fantastic creatures and their lore. Because you are 14 years old and you're excited by your new Christmas present and you can't wait to get back to school and talk to your friends about this thing. Because your roommate left it on the coffee table and you were bored one night. Because your new girlfriend seems REALLY into this.
The MM isn't just a reference book. It's also a window into the worlds of fantasy and imagination that these stories unlock. Why can't it be a window into all the awesome D&D stories that are out there, and some new ones to boot?