Movies show Americans declaring their independence from Britain, taming and settling the Wild West, and winning two world wars. Why? Because these things are (more or less) true and moviegoers know it.
No movie has ever portrayed George Washington, Thomas Jefferson, Abraham Lincoln, or Teddy Roosevelt as a savage spearchucker. Americans would be outraged if a movie made such a claim. But hundreds if not thousands of movies have shown Natives as savage spearchuckers and no one blinks an eye.
Movies tell the (American) truth
Given all this, why would Americans think movies are falsifying Native cultures and religions? There's no good reason to think movies are telling the truth about Americans but lies about Native Americans. Every history-based movie purports to tell a version of the "truth." So moviegoers respond rationally and believe what they see.
They do this for several reasons: Because they have no other sources of unbiased information. Because everything they see in the media reinforces their beliefs. Because movies are based on historical research, books, and expertise. And because respected filmmakers (Gibson, Spielberg, McMurtry and Ossana) vouch for their work's accuracy.
Below is a plausible but phony mural from Apocalypto. What percent of people would believe Mel Gibson filmed an actual mural and what percent would guess he fabricated this mural for his movie? I'm guessing the ratio would be 80%-20%.
Now let's add a note from a published article, which I reported in Gibson Sacrificed the Maya:
Hansen defends the film, believing that his fellow Mayanists will be "pleasantly surprised." He says, "For the most part it is very accurate," and "I was amazed at the level of detail, the stone tools, gourds, iguana skins, strung up turkeys, just amazed."
For more on the subject, see Educating Russ About Historical Accuracy.