I'm not saying it makes no difference, but it doesn't make a $15M difference and while it means a lot to guys like us that make 100k a year, no free agent that I have ever heard of has ever made a decision to go to a team because of there being no state income tax.
Let's say there is a difference in 35 games per year of playing in a state with no income tax. Let's say Dwight makes $20M per year. $20M/82 equals $243,902 per game. The highest state income tax is California at 8% so let's just pretend all State income taxes were that. 243,902 x .08 equals $19,512. $19,512 in taxes times the 35 game difference per year equals $682,926 per year. Over 4 years, that is a difference of $2.7M, not $15M like you claimed. And it wouldn't even be $2.7 because not all states have nearly as high of rate as California.
Again, it's not insignificant, but when you are making a quarter of a million bucks a game, you aren't going to let 10 games worth of salary over the course of 4 years make your decision.