Ok. Let's think this through for a minute.
How exactly does the NHL enforce a system that is based on net income, not base salary? Let's say they implement this right now given the current political environment. The Canadian and high tax states get a bit of extra room to work with, so let's just say they are allocated an extra 20% cushion above the base US cap for states not affected by state income tax.
Toronto goes out and signs player X given this 20% cushion. He would have made $10 million in Florida, so they sign him to a 7 year, $12 million AAV contract.
Simultaneously, Tampa signs his equally gifted twin (Player Y) to a 7 year, $10 million contract.
The next year, a high-tax candidate is elected President of the United States. Federal income tax is raised 20%. Due to the base measure being increased, the 20% cushion is taken away from the Maple Leafs, or Tampa is given the same 20% cushion to work with. However, the salaries are set. Suddenly, Toronto is looking at a massive contract without the cushion, or Tampa is given extra cushion to work with, despite having already received the benefit of signing Player Y at a $2 million discount.
In year 3, the high-tax President is impeached and taxes are slashed in the US, 40% across the board (large, whole numbers for simplicity). Toronto is given an even larger cushion to work with, or Tampa's salary cap is slashed despite giving out a contract based on the past political climate.
During this time period, various local elections are occurring, state tax codes are being rewritten, and the political landscape marches on.
How exactly do you propose being a GM in this league? How will you ever give out a long term contract? How do you plan for political changes that are suddenly affecting how much cap space you have to work with?
All the while, you entice players to sign with you in your 'big market' team with endorsement deals that grossly affect your net income, that the NHL doesnt regulate.