Looking back on it, my mom got free rent due to where she worked (albeit, a $377/mo check that barely supported us). So, when she found out someone was living on the property, she invited them in as she is a strong Christian and wanted to help. She didn't know them, well enough, that is, and could've put us all at risk. It turns out that they weren't a bad person, but a person who just lacked motivation. They stayed with us for 6 months, trying to find work, but could never land anything, nothing at all.
While the concept of helping others is great, some people just can't be helped, even the good ones, without a larger support system than just free housing.
Going back to the story of the homeless person that lived with us, we could've been put in a very bad situation ourselves, if our state (Washington State) had turned too progressive then. They would've been able to establish residency, and my mom wouldn't have been able to get them to move out at all; it would've had to include the owner of the property, and that strain could've put my mom out of work.
So, why not let someone live with you to help them out? A lot of reasons. Nowadays, you can't just evict someone if they choose not to help themselves in some states. That would be a horrible situation to be in: Inviting someone in with good intentions on helping them out and not being able to get rid of them for a year or more, all the while they are free to come and go as they please, and do whatever they want inside the home, for as long as they aren't harming you.