“Writers Forum” or “Writers’ Forum”

At first, I thought this is an easy question to answer, but upon a Google search for "writers forum", I realized that both are used commonly.

I did a quick search of some grammar sites and this is what I found:
Generally, when using a noun as an adjective, it should be singular, not plural. A possessive noun can also be used, but with a different shade of meaning.
We say, vegetable soup, hen house, bean burrito, horse stable, etc. no matter how many vegetables, hens, beans, or horses are present. While unusual, you could say hens' house (etc.) meaning either a house of hens or a house that belongs to a group of hens.
Thus, "writer forum" or "writers' forum" would be acceptable, but writers forum would probably be incorrect. If it is a type of forum then writer. If it is a forum belonging to or made up of writers then writers'.
Writers forum does seem to be commonly used, so I wonder if there is some rule I'm missing or if it's just a lot of laziness.

Porsche- I think you're right. And the commonality of "writers forum," in my opinion, is an artifact of the English spelling convention of using an apostrophe to indicated the genetive case. As we all know, spelling conventions are just that have nothing to do with language per se. I could go on and on here about how spelling influences language and vice versa (maybe someday, it will be considered correct to use "writers" and "writer's" interchangeably, as the two are phonologically identical and therefore, in speech, the difference needs to be inferred from syntactic context) but I won't because I think it's pretty clear.

Consider also "veterans' home" and "veterans home." I've seen both used in the titles of these institutions.

Hmm..."Writer Forum" sounds *really* weird to me. "Fiction Forum", or "Writing Forum"--even "Shakespeare Forum"--sure, but using a non-posessive noun with "forum" makes it sound like the noun is the *subject* of the forum, not a description of a generic participant.

All very interesting, but can spelling come into this. She spelt `their´ wrong, didn´t anyone ´notice this, or isn´t it important. ?

Is theire a prise for the post without typeos - appostrophe or no apostropheee, thats thu kwestion...interesting or not interesting, overwhelmingly no.