Internet apps without internet


I saw on another thread that it is possible to access Youtube and Netflix via the Optik apps even without internet service.


My uncle does not have internet so when I was over his place a couple of nights ago, I gave it a try.  Sure enough, the Youtube app started without any apparent issues.  I did not go further than Youtube's home page because I was uncertain of the ramifications to his bill.


My question, therefore, are there any costs associated with accessing Youtube when one ostensibly doesn't have internet service?