Exercising control is not necessarily about control. At the most basic level, exercising control can be secondary to some other purpose, in which case exercising control is "about" whatever that purpose is.
More broadly, as I pointed out previously, the concept of control is freighted with complex connotations. It doesn't take much to switch from a positive or neutral connotation to a negative one when discussing control. In particular, anything that touches on control being an end in its own right (e.g. calling something "about control," suggesting that achieving control is a primary motivation) is going to carry a negative connotation for many people, whereas saying someone is "exercising control" can more easily carry a neutral connotation of doing a job, or even a positive connotation of satisfying a responsibility. Since "exercising control" and "about control" have incompatible connotations, they can't be interchangeable.