Some would argue that the existence of a hard cap in the NFL is part and parcel of their success as a league. It's not about "fair" or not...it's about the league trying to get as many franchises involved in the post-season (or post-season hunt) as possible, to elevate the value of all franchises. There's no universally agreed-upon magic percentage of league revenue that should or shouldn't go to player salaries; that's a made-up construct that has no relationship to the business of sport. I mean, if Ronnie Brown or Wes Welker or whoever feel they are underpaid, and they can make more money doing whatever is their next-best option to playing football, they are free to do that instead of play football for a living. The fact of the matter is that everyone in the league can make more money playing football than they could doing anything else. I'm not arguing about whether their take is fair or not because everyone's line is different there, I'm just saying the NFL makes their market how they make their market, and the success of the league is testament to how well they do that.
The Yankees have done that (steal young talent from other teams) throughout the better part of the last 2 decades, and they did win multiple series over that time frame as a result. And baseball does have a pretty direct correlation between team payroll and post-season participation (with the A's being one of a handful of notable exceptions). You can argue whether that is good or bad, I think I'm sort of neutral on the issue.
I do agree no one forces a team like Seattle to back up the Brink's truck to sign Robby Cano, but it's a bit different than the Yanks signing A-Rod or Clemens or Tanaka or whoever. The Yankees can afford to be wrong about guys they sign to huge contracts, whereas smaller market teams have a lesser margin for error. If salaries are squeezed into a tighter box, those types of astronomical numbers never get reached because teams simply can't afford to fill out the rest of their rosters if they have so much invested in one guy, consequently it doesn't cost that much to sign a single player. But these are all old arguments you've all heard before and likely won't change anyone's mind because there are myriad reasonable counter-arguments to everything I said.
I will say this for the lack of a salary cap: it's the truest expression of free market economics in sport. Publilius Syrus' pithy words of wisdom never rang truer than in the baseball free agency market...(maybe with the exception of Carl Pavano).