Well, I feel it is relevant. Would the teams have a say in any salary cap ruling? If they have any kind of sway there is no way they would agree to it, especially when their television and sponsorship income will be at direct risk.
Take away the risk of relegation, and thus losing the vast majority of your income stream then a salary cap would be far more 'acceptable.' Who needs to go for broke to attract the best players when the rug will not be pulled from under you.