I want to have time to find all the threads in the last year where this idea has been lobbed out and blown out of the sky, but let's see if I can just hit the high points here:
1. No other professional sports league with a salary cap system considers taxes in various jurisdictions when setting the cap for each team; all teams have the same cap. Why should the NHL be different?
2. Which taxes are we talking about? Just pure income taxes, or are we going to consider property taxes, sales taxes, and other such taxes and assessments [including, but not limited to, SSI/Medicare payroll taxes in the U.S. and any similar counterpart in Canada]? If we're only considering certain taxes, why aren't others being considered - especially if their differences are as large [or larger] between certain areas? With respect to income taxes, tax rates can [do] vary by level of income; it may not be particularly material at higher levels, but it may be material when considering salary vs. bonuses. How do the "correct" tax rates get calculated - by using actual data as it exists [actual salary vs. actual bonuses], by treating all team payments to players as salary, or by using some estimated split? Why should that approach be used?
3. Are we only talking federal and state/province taxes, or are we including all the municipality taxes? And what municipalities - just the "primary" cities where each team operates, or those of the surrounding areas where players actually live and where tax impacts may be more or less favorable?
4. Are we factoring in the impact of #3 on teams who play games in away cities that levy taxes on players, or not? If not, why not? If so, how does that get treated when division rivals may not play the same number of home/away games each year or even year-to-year? Do we have an "average" cost based on "average" games played in a location, or do we explicitly calculate that based on whatever the schedule happens to be?
5. What if a tax change is passed that's applied retroactively to July 1 or prior, after the league year has started? Should the cap be re-calculated for everyone? If the answer is 'no' and the impact of the change is material [say, at least equal to 50% of the league minimum salary], why not?
6. What about cost-of-living differences between cities? Shouldn't that be considered, since two areas with similar tax rates could have a different cost of living that could impact the purchasing power of a player's after-tax income? If so, what components are going to be considered in calculating that cost-of-living difference? If not, why not?
And I'm not even scratching who's going to collect this data, build the database to house it, do the work necessary to calculate an "after-tax" cap, and administer it and keep it up-to-date.
That's just sitting here off the top of my head in about 15 minutes and expanding as needed. I guarantee there's other considerations that didn't come to mind, more elaboration that could be done on the above points, and others points that others have mentioned and I can't recall. Put bluntly: even attempting to "fix" the "problem" of differing tax rates between franchise locations is a logistical nightmare and no matter what, someone is still going to complain that things still aren't fair because [insert reason]. Which brings me back to the lesson I keep trying to teach some people: life isn't fair.