I'm a landlord and my wife works in property management. Here's why this is a big deal:

You're technically violating contract law and opening yourself and the landloard up to a ton of liability by doing this. Leasing and renting is much more complex than simply letting someone sleep over at your property. A property owner can get in some serious trouble if something were to happen (someone gets hurt, robbed, etc..) and he/she hasn't taken the correct steps to protect his/herself. Not to mention the issues around covenants, local ordinances/regulations, taxes, mortgages, and insurance.