Why the Bass model fits without decision variables
Bass, F., Krishnan, T., Jain, D. (1994)
Marketing Science; Vol. 13; No. 3; pp203-223DOI: 10.1287/mksc.13.3.203
Describes a Generalised Bass Model (GBM) that takes account of price changes, advertising efforts, changes in target market size etc. These are defined as decision variables and the paper describes a model to handle them.
Under the special case where there are no decision variables, this Generalised Bass Model reduces to the Bass Model.