Sorry, I'm a relative newcomer here and only caught up with this thread in its revived state.
The reason for the wording of the sign on Aguirre's door, I believe, is because Ang Lee and Larry McMurtry came across a similar sign when traveling together to look at various locations in the Western States. Here is how Ang Lee describes it:
Lee: The location, we have seen it. We have traveled through it or [seen it in] a movie. I think it was more the people. Of course, they are always nice people. They are just like everybody here, except they are nicer, and I felt guilty that I was going to do a tough movie about them. [Laughs.] Something struck me, [with my] fresh eyes. Sorry to say—[the] eccentric[ity]. You go to a bar and just see the things that they hang up. I can't believe the things they hang up inside and outside of bars. So imaginary. I don't know how else to put it. Of course that will annoy a lot of Westerners, because [they'll say], “We have a lot of normal people.” There are two sides. One is like that. It's like [they] don't want [you] to go into their territory. I saw a sign in Wyoming—a “No Trespassing” sign. And it says, “Trespassers will be shot. Survivors will be shot again.” [Laughs.] Some characters are like that. If they hear you're coming with the Wyoming film commissioners, [they want] nothing to do with the state.
This extract forms part of an interview that Ang Lee gave AboutFilm in the December of 2005.