I actually had an idea on why they do that last year. The dark coloration and tendency to work the tendrils into shaded spots might be an adaptation to prevent algae growth inside of the pitcher. All of the dissolved nutrients inside of the pitcher would cause an algae bloom if exposed to enough light. This actually happened to one of mine once. Said algae would rob the plant of its carefully acquired nutrition. The upper pitchers probably have something in the fluid that prevents algae from growing.
I should have been a bio major.