Some years back, Justin and I had
a similar experience at a
TX State Park after a holiday weekend, and a similar reaction to it. You'd think the very same beauty, nature, etc. that brings people out to a park would be what would motivate them to not trash the place, but sadly that assumption does not apply to 100% of park guests.
I think the majority of park guests are responsible and pack out what they brought in (if not collecting previous guest's mess, as well). Sadly, it only takes a few irresponsible and/or selfish people to make a thorough mess.