They can't walk in, wipe their shoes on the hallway rug, make a pot of coffee, use the bathroom, turn on the TV, and take a nap on the couch. At least not without their tenant's invitation.
When they chose to rent out the house they yielded some of their property rights. The old landlord argument that "it's my house I should be able to XYZ" doesn't hold water.
Why should landlords have that expectation? I think the default case should be that when someone rents a space they have freedom to do what they want with that space until they stop renting it, and then when they stop renting it they must be forced to return it to its original condition.
Did you know in Australia it's normal to give your landlord a tour of your house every 3 months to prove you haven't broken it? That's completely ridiculous.