Why is it a lunacy to rent out your spare unit, but not a lunacy that an agreement between two adults somehow is going to lead to someone else using your proper

Why is it a lunacy to rent out your spare unit, but not a lunacy that an agreement between two adults somehow is going to lead to someone else using your property for free indefinitely? The ironclad assurance is a legal system, that allows you to evict when one side breaks the agreement, which failed ridiculously in this case.