Airlines have what is called a contract of carriage.
Not sure what airline you are flying but if it it DL the
CoC reads in part (the same language will exist for most airlines)
B) Failure To Occupy Space
If the passenger fails to occupy space which has been reserved for him/her on a Delta flight and Delta fails to receive notice of the cancellation of such reservation prior to the departure of such flight, or if Delta cancels the reservation of any passenger in accordance with any provision of this rule, Delta will cancel all reservations held by such passenger on Delta flights for continuing or return space, provided Delta originally reserved that space.
And the rock and hard place difference is that if you notify them they will charge you the difference in the fare