If you don't fly the return you forfeit the service and any value it may represent (there is a value associated to the return as per the fare rules).
Flying only one-way of a cheap r/t is not the same as buying a one-way - as the fare conditions will differ completely. E.g. check date change and cancel fees for a cheap r/t vs a o/w and you will see these are not the same product.
It states exactly in the fare rules what will happen when you don't make the return flight - you lose it and any remaining value.
In short - you do accept the fare rules when opting for a cheap r/t.