Here's the reason why:
The theme of the movie is dreams, and the creation of dreams.
The theater represents a dream-like state that you go in.
The top not falling down is important to the theme of the movie. Because you don't know the ending, you end up thinking/dreaming about what what you think the ending of the movie would/should be.
And that's why it's called Inception, because the director, Christopher Nolan, plants in your brain the beginning of a dream that will then take on a life of its own. Your dream.