I agree completely. Again, I wasn't criticizing the way the Vikings have done business, or at least I wasn't trying to. I don't live in the area so I don't know all the details about what they can and cannot do in terms of generating extra revenue.
I believe the simple solution is to base the salary cap and floor solely on league shared revenue. TV and radio deals, NFL Pro-Shop, gate receipts, etc. Anything privately generated stays with the owners themselves. This levels the playing field and keeps the salary cap from exploding every time Jerry Jones figures out how to squeeze another dime out of Dallas fans.
Of course the one complication with this plan is the anti-trust exemption the league currently enjoys. The leagues stance is they're one business made up of 32 parts, and as such, ALL revenues are shared amongst the teams. If they take the approach I laid out, they would be admitting they are not ONE business made up of 32 parts, but are instead 32 separate business. This view point would impact the draft and would kill the salary cap as implementing those things would be collusion as is a violation of the law. Unless the NFL can get yet another exemption, like they got for the TV rights. In this day and age, I don't see that happening.