Problems with this idea:
1. It would be an absolute nightmare to adjust salary caps based on income tax, that's by far the biggest issue with this idea.
2. People who pay higher income taxes get some of that back when the city spends more money on public works, it's not like the money just disappears and you're comparing 2 equally good locations. The one with the higher taxes is going to have better public works.
3. At least in the NHL's case, places with higher income taxes likely have more opportunities for sponsorships and other off ice deals
4. Cost of living in different cities is different.
5. Money isn't everything with players, and for who the discussion is actually about, do you seriously think they'd pick one city over the other if they'll be making like $8 million regardless?
6. I also would imagine that some players use other addresses other than their current address for tax purposes. If your parents lived in rural NY and you now play for the Rangers, I wouldn't be surprised for a player to list his parents address on his W-4 forms.
This just sounds like sour fans of Canadian teams, mostly Leafs fans, that want an unfair advantage to attract guys like Stamkos to join their team.