Sleepovers are a recipe for disaster, especially if you don't know the parents. It's so much easier to host things in your house where you know nobody is crazy, but when you drop your kids off somewhere else, there's no guarantee everything will be okay. When I was a kid, I slept over at my friend's house, and frequently, her parents would take me to the local megachurch with them without asking my parent's permission. It's one thing to have a household policy of "If you're sleeping over on Saturday night, you must go to church with us on Sunday," but it's a whole other thing not to tell the parents that that's the expectation.
One mother recently found herself in a far more horrifying sleepover situation than that. Her son was sleeping at a friend's house, and when he called her asking her to pick him up, she was horrified to hear why.