At what point did game designers go from "Guys who want to make my game better" to "Those bastards who are trying to ruin MY game!"?
Not necessarily any point - your question is based on a premise that may not be valid. Gygax didn't say "you can't use guns in your game", though it depends on how you interpret the phrase "Official ADnD game worlds". The 1E DMG has rules for running DnD characters within a Boot Hill game world, AFAICT this was so you could transport DnD characters to Boot Hill or vice versa. Don't forget the Murlynd NPC, Barrier Peaks module, etc. IMO there's no logical reason to think that the context your giving the statement is accurate for what was intended.
Secondly, *had* Gygax meant what you're saying he meant, there was no internet to complain to. Maybe someone could have written something to the Letters forum in Dragon, but they were probably too busy arguing about evil PCs.
Thirdly, people yik-yak much more about "game design" like RPGs are some sort of science. I suppose this is to lend some credibility to what otherwise seems like a matter of opinion. In the old days IME someone like Gygax could say "don't use critical tables" and everyone would pretty much ignore it. I could write about using critical tables in my game on Usenet and not have to hear 1000 fan-boys rail at me about how I was ignoring - not just Gygax - but somehow violating natural law (or "good game design" or however they're framing their opinions these days). But now try doing the same thing with some sacred cow in 4E, like wealth-level values. "OMG - your PCs don't have the recommanded [sic] magic item wealth for their level!? Why that very well could knock Earth out of it's orbit!"
So as a consequence, when people don't think for themselves, and assume that individual DMs will apply appropriate judgement in their games, then it falls upon authority figures, like Mearles, to keep us all in line. With this increasing (and inappropriate IMO) reliance on authority figures, then what they say takes on inflated significance, and thus what could be construed as just some gamer's (however knowledgeable) opinion suddenly is now a point of potentially bitter contention.