Never had a problem bringing things into the park. Water bottles definitely OK, it says it somewhere on the website. No glass, I'm sure they would say something and I totally understand that rule!
I've brought several small "snack" things and they didn't even blink. I think they are more looking for weapons or large amounts of food.
The idea is for the park to make money on the food and I think if they had to clean up after a family of 6 (times a 1,000) with their trash, bottles, straws etc., it would probably take more staff to clean up. Also, it takes tables away from the patrons spending $10 on a small hamburger...yikes!!
I know one day I was pretty peeved about just spending $56, looking around for a table while my food was promptly getting cold....and finding a mom with 2 kids using a booth to feed them baggies of cheerios she brought in for FREE!
I eat the things on bring on the go, at a bench or quickly in line. Also I've actually packed a large cooler and left it in the car....there's is no problem going to your car and eating, I've never been hassled for that.