Okay, so for everyone who's been wondering:
How did this work? How does this work?
Well, first, unlike Dumbledore, Snape didn't breath a sigh of relief when Lord Voldemort disappeared. He's been busy, and this is his project. It's his project both because he actually cares about abused kids, and because they're reasonably loyal people to those who aid them. In short, by giving these kids solace, he's building up a network of people that are loyal to him (and will do what he asks, when he asks).
Because, apparently, no one else cares about these kids.
Snape uses kids as spies/watchers, what have you, trained to recognize when kids have that look of child abuse or neglect. If they aren't in Slytherin already, they do basically get kidnapped into Sanctuary, though it's not generally that traumatic. Learning that other people have been through this, and have survived, is honestly good for most of the kids.
(Harry, incidentally, would have gotten a much more ... thorough... welcoming if he wasn't Harry Potter, In Need Of Guarding So He Doesn't Get Swarmed).
Snape's been doing this since he first started teaching at Hogwarts, so there's a lot of graduates as well. (I kind of meant to have a scene about the havoc and mayhem just having that many Slytherins in the school was causing, but... Rest assured, Ron was upset!)
It is possible to get kicked out, but that's mainly reserved for shites who can't learn their lesson about lashing out and hurting other people that are hurting.
Children who actually were sorted into Slytherin can also be kicked out - but for them, it's like the gloves have been pulled off, as Snape's rules on Slytherins Stick Together no longer apply.
... there are kids in Slytherin who do torture cats and other small animals to death. They get kicked out. Not common, but they do exist. There is a reason for Slytherin's bad reputation, even if it's mostly undeserved (striking back when struck at is something a Hufflepuff would do just the same!).
If one's sorted into Slytherin, one pretty much gets an automatic pass to be in, but... it's going to feel a bit awkward if you don't have any scars to show off.
This, in the main, is my "They Spent 10 Years doing NOTHING?" Rant, in story form. I hope you enjoyed, and I hope you'll find it in your heart to write me a review. Thank you very much for reading, and really, any criticism is welcomed!