I'm going to disagree. Carrying the games of a home team creates a revenue stream that the affiliate would not have otherwise. Once the passion of a local fan base is added to the mix, advertising sales become based much less on the numbers (cost per rating point, cost per thousand viewers) and much more visceral. The affiliates can charge more regardless of ratings based on an increased demand. And that's a fact. And facts can be used to prove
And you're right, from what I know the nets do lose money on the NFL. Although, I'd have to be an idiot to believe that they don't make it up somehow. A major reason the networks spend billions of dollars on the NFL is to promote their other programming. A home team in the country's second biggest market (assuming increased ratings) would provide a much better outlet for promotion. If the networks are whining about a team in LA it is simply posturing. Overall it makes good business sense.
Then you have to wonder why an owner in another market, say New Orleans or Buffalo, with about one tenth the population of LA wouldn't want to move their team. Think about it. While everyone in the NFL is profitable, teams in big markets are making a lot more money than teams in small markets. Why? Local media deals, luxury boxes, and corporate sponsorship are a few reasons. These are worth much more money in LA than in Buffalo.
One more thing. Yes, the NFL has used LA to leverage new deals in other cities. However, a team moving to LA will not stop this. It will only fuel it. People in Buffalo/New Orleans/Jacksonville/whatever will feel even more vulnerable. "Hey it happened to New Orleans/Buffalo/Jacksonville, it could happen to us" will be the prevailing thinking. And there's plenty of markets that'll take one of these teams. Portland, San Antonio, Vegas, or others will all line up to put the pressure on the market that won't pony up.
It's a tangled web.