I'm wondering why people don't charge rent/room instead of charging rent / entire house. 

Most states give 90 days before auction - at which time you continue to charge rent and late fees, then sell their stuff to cover the cost.  

Either which way how are the parents going to charge rent to the friends if they are only 1 room let alone efficiencies? 

I had to install smoke/CO detectors with non-removable batteries before selling my condo in CA.

So the boxes could have caused these minimum clearances to not be met.But as Rob K has said, it's going to be hard to prove that was the exact cause.Due to lack of common sense in many people - including homeowners (not just tenants) - I have some documents that I supply with the lease to explain some of the simplest matters that normally fall under common sense - like changing batteries in the alarms :)

Batteries from China are not that great, so you can get batteries from here.  

A very simple device, current from a 9v battery makes contact with water and sets of a siren.

For example, a 1 unit: 2 bedroom 1 bath apartment for 50k, and charge rent for the one unit while not controlling the whole complex? 

In some states if an apartment isn't habitable and the tenant has to temporarily move out then you're not allowed to charge rent for those days.   

I have a few Yale Assure 2's, and the battery life is $hit.