This is definitively not a fact. In the hotels in which I worked, the extra person charge did not cover the cost of a rollaway bed. The rollaway always was an additional charge (and based on availability).

Additionally, no one should assume that a rollaway bed is an option without talking to the hotel prior to arrival. Some properties don't have them. Other hotels are legally prohibited from some rooms due to fire codes.

ORDroad: As phalkon30 and Dave Noble suggest, you really should be asking the hotel to answer your questions.