Why do people insist a person cannot be a legal tenant without being on the lease?

Lax landlords will give verbal permission for a person to live in a rental. Some never have a written lease. For example, I know someone that rented from his aunt and NEVER had a written lease. But they are LEGAL tenants on LEGAL month to month verbal lease. I have known of landlords giving verbal permission... show more Lax landlords will give verbal permission for a person to live in a rental. Some never have a written lease.

For example, I know someone that rented from his aunt and NEVER had a written lease. But they are LEGAL tenants on LEGAL month to month verbal lease.

I have known of landlords giving verbal permission for a roommate to move in...sure they are not protected by a written lease, but they are still LEGAL tenants eligible for and have access to all amenities.
Update: answers miss the point, yes lack of written lease including all occupants is STUPID, but does not make the tenant ILLEGAL.
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