Rats and mice are rodents, they are considered vermin and pests. They are legal to kill and the government and humane society do not care how you choose to do it. They are pests of the highest order and must be exterminated with extreme prejudges. Squirrels however are nothing like mice and rats. For one, they do not live in your home permanently and they do not interact with humans, they do not want human food unless it is outside and left unattended but they will never actively enter a human home to live there. They far prefer the attic for its seclusion and privacy. They must protect their babies from other squirrels and predators so the attic is the perfect location.
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When mice move into your home they have no intention of leaving. They will stay in your home forever, generation after generation, hunting for food in your kitchen, and living in the walls. Over time they will move up through the walls, floor by floor, and then eventually enter the attic, once they are up there it becomes very hard to exterminate them. Most companies use commercial grade rodenticide which dehydrates the mice and allows them to safely die in the walls where they will not be a biohazard or threat to your family. This is a common mistake people make in thinking dead mice in the walls is a danger, it is not.
Rats however do not live permanently in the home but do live permanently on the property. They will live in the yard and eat whatever they can find and then enter the house to find more food and materials for making nests. They will raise their children outside and live outside but use your home as a grocery store. If you have rats in your home you will have to use snap traps to kill them because unlike mice rats are very toxic and have very high water content. They will rot and smell and make a stain on the wall. They could infect the whole house with the plague or worse. Their dropping is highly dangerous and if you have them you will need to call the exterminators to get them out.
It is clear that mice, rats, and squirrels are all very different and provide different levels of threat to a home. Squirrels live in the attic only for a couple of months while mice live in the walls forever, rats will live in your home, or outside of it, wherever they can find a good source of water.