Just a question of accounting...
For salary cap purposes, I believe that a players salary incudes any sponsorship and image rights.
I don't really know much about those, but how can a club control them?
If I watch wasps Twitter updates, we were still announcing sponsorship for some players last month.
If we'd won the world cup and Nike suddenly wanted to sponsor Maro Itoje for £1m per year, how does that get squared?
Whether you agree with the salary cap or not, it seems unfair to stop a player earning the sponsorship or image rights available to them
(I know that it's to stop unscrupulous clubs bumping a players salary through sponsorship)