It's also the rule in Yosemite. There are a lot of reasons to comply with it regardless of the ick factor.

I comply with it in the best interests of sanitation and health. Waterways in Yosemite test positive for e coli, and 10-15 feet off the sides of the heaviest use trails, you are surrounded by the toilet paper of 10,000,000 tourists who have gone before.

If you are using toilet paper you buy in the grocery store, that's not really paper - it's cotton fabric. It doesn't biodegrade well at all without all the chemistry they use in sewage treatment. For paper that's easier on the environment get toilet paper designed for RVs or specifically for backpacking.