Okay, they can't burn the plant with fire using alcohol as an accellerant, but salting the earth somehow works against it? And how has the plant remained so isolated for this long, considering its super-fast growth and sentience!?
Salting the earth prevents other plants from growing up there and getting eaten by the vines and probably prevents it from taking root outside the pyramid. You could ask the last sentence for a lot of horror premises.
most plants won't grow in salty soil. That's where the phrase "salting the earth" comes from, where when you really want to screw your enemies in war, you salt the earth so that they can never grow crops there again. And considering what happens when you burn poison ivy, setting fire to that thing might lead to a Return of the Living Dead ending.
Burning plants can also cause the seeds or spores that it reproduces through to float over long distances allowing the plant to spread
What was keeping these plants in check before humans showed up? They don't seem to have any predators, and probably no parasites or diseases either.
No explanation is ever given for what exactly the Plant is, which means that it could be a supernatural creature, which would explain a LOT of this.
Nothing? The Mayan tribe was there longer than the plant was. I mean, since the temple was there before the plant, it stands to reason.
In the book, at least, it's suggested that the Mayans have a truce with the plant. They don't try to kill it, and it's happy to stay on its hill, with the salted dirt surrounding it to make sure that it keeps up its end of the bargain. In the movie, there's a possibility that they could worship the plant, since they've apparently built a pyramid up around it, so that would explain why they haven't tried to kill it.
If the natives know how dangerous the plant is why would they take those kids to the ruins in the first place?
They didn't. The yuppies (carrying that Idiot Ball proudly) stumble upon the pyramid by accident. The natives run out of the jungle, screaming at them to get away from there. They naturally run up the pyramid, rather than staying put like they're told.
The cab driver tried to warn them away, but gave in when they insisted. And to be fair, they couldn't understand the guardians, who were speaking some native language other than Spanish.
So, why exactly did they immediately quarantine the stupid tourists? None of them were carrying the plant, yet. Besides, what do they care? So a few stupid Americans take a man-eating plant home. Serves them right. Let the American government take care of that. Better than holding them hostage in the death pyramid.
They may not be total Jerk Ass tribesmen or whatever they are, perhaps? Also, consider the above ideas that The natives may either worship the vine as some sort of supernatural being or have some sort of truce with it. If they worship it, they may see it as a sort of offering in that - once it touches the pyramid, it belongs to the plant. This may also be one of the terms assumed to be part of the 'truce.' Really, there just isn't enough information to tell, but it COULD have an explanation. That may be, in fact, a point.
The feeling I got from the book was that the native people were more interested in keeping the plant where it was. They definitely did not worship it. The spores were also said to be very sticky, such that anything it touched (especially clothing) could not be cleaned enough to get the spores off. So once Amy (in the book) was touched by the plant, she had spores on her. The native people knew the others would try to get her out, so they were all forced into quarantine. No idea how it was in the movie.
Look how it spreads. Ever hear of a movie called The Thing (1982)? You have to do what it takes to keep an infection like that contained. Sucks to be them.