He was sleep deprived and sugared up - of course your ds had a meltdown on the way home. It wasn't really about your strict approach to parenting, or the differences between there and home, that was just what he said in his half-delirious state as he crashed off the sugar. I would use this experience as a way to start teaching the value of self-discipline. Your ds doesn't want to feel like that every morning. I wouldn't address the content of his tantrum at all - the fact that the situation made him feel so yucky is a great entry point into discussing good decision making and taking care of yourself.
I remember staying up till 2 am, playing super mario brothers, and eating a whole box of good humnor bars. Good times!!!!!
Growing up, I was an only child with the calm quiet house that was pristine. My BFF was one of four in a big NY Italian family. There were usaully at least 8 kids over at a time at her house. I loved going to her house, she loved coming to mine beacse she said it was so quiet clean, and she had privacy and attention from my mom. So the friend may be saying the same thing to his mom about your house.
These are fun, innocent memories. As long as the environment is safe (mom watching kids from the window when playing outside) etc. I would be fine with it. We are very strict with food at our house , but my kids know that when they go out somewhere else without me, it is their choice. They have learned that a whole bag of cheetos before bed is not a good thing.
BTW I do not even try reasoning with a child after a sleepover. I usually just ignore DD the day after she come back. She is usually happy to read and nap all day.
I do not think I allowed sleepovers until 8 years old, so if you are not comfortable yet, do not feel in the minority just on an age based issue. Having said that, sometimes "Let them Eat Cake!"
My youngest DD has severe food allergies, and I only wish I could let her have a candy bar for breakfast just because that would mean she did not have the autimmune disease she has.
Sounds like there are two issues. The first is that you might not feel like your son is safe over there. I think that feeling is worth respecting and exploring. If you truly do not trust the family with your son, do not let him sleepover. You might not even want to let him have playdates there if you are truly worried that he is not safe in the house/neighborhood. I would never send my DD somewhere that I was concerned about her safety, especially at age 6.
With respect to the post-sleepover crash, that's really common, and parents have to know whether they can handle a super cranky kid the day after the sleepover or not. I don't like it either, but I know it's worth the fun, every once in a while. But seriously, if you don't like it, no one will blame you for putting off the sleepovers for a few years. Many of our DD's 7-yr-old friends are still not allowed to have sleepovers, so it's no biggie.
About your son's friend's house and the freedom, that is really interesting. There's always a grass is greener thing going on for kids. I know I much preferred going to my cousin's house because she got to watch trashy TV and eat in the living room and all sorts of things that were mortal sins at our house. How you and he deal with it is the issue. I would just make it clear that your house your rules and if he's going to come home and whine and complain, it will be a long time before the next sleepover. But I'm a bit of a hard ass myself.