As I said, it exists as a separate regime for the designated list of professionals, and it's important to keep in mind that the general rules apply not just to industry generally but to other classes of professionals who for many years—decades, even—have managed to make estimations of what the fair market value of the work in progress is and have been able to work within those rules.
There's guidance both from international accounting standards and the Canada Revenue Agency on how to appropriately apply accrual accounting to fairly specific fact situations. It essentially puts these excluded groups on the same playing field as others who have been able to comply with the rules for quite some time.