Some of what seems to be in question here is the matter of who is providing what value to whom.
It has been accurately pointed out that the spot does provide the entertainer with an opportunity to build up their brand. That's why they do it for free, also, performers are performers because we like to have people looking at us while we do our thing, and the Super Bowl halftime show is one of the biggest opportunities for that.
However, it has not always been this way.
Those of us old enough to remember can think back 25 years ago to SuperBowl halftime headliner "Elvis Presto," who was, and I am not making this up, a former Solid Gold dancer who did his hair up in a pomp, gyrated around with a broken 12-string, and did perhaps the lamest "magic trick" ever.
How we got the superstars was that the NFL and the broadcasters finally figured out (partly because of the complaints about the lameness of the Elvis Presto act: it was only 4 years before the act was Michael Jackson) that they needed someone to keep people tuned into the game in case of a blow-out.
In the case of a big blow-out, people would just turn off the TV or switch to something else, just like they leave before the last inning at a baseball game, or before the clock has completely run out in other sports if the team they support is getting their asses whipped.
Also, after Elvis Presto, the other networks picked up on the idea of Super Bowl halftime show lameness and started putting stuff against it to get viewers' attention.
If the networks can't keep people watching the show, they can't sell the commercials for as much.
By keeping people tuned in through the end of the halftime show, the appearances of these huge acts makes millions more for the networks (and by extension the organization that sells the broadcast rights to the networks).
It makes sense from one standpoint to charge money for people to appear, because the appearance will put more money in their pockets, and for this same reason it might seem to make business sense for these entertainers to pony up.
Where it DOES NOT make sense is that it inevitably hurts the brands of both the show and the entertainers.
And I do not appeal to morality or ethics with these arguments, rather, it winds up being bad business. Here's why:
Part of the glamour of that level of entertainment is the (often illusory) notion that the performers lead fabulously wealthy lives and make millions per show. If they are seen as calculating and desperate, it cheapens that glamour.
Similarly, the current "story" of the Super Bowl halftime show is that they are going to have someone who is completely at the top of their game, the best entertainment with the widest appeal. To be selected to play the Super Bowl is a great honor, only bestowed upon the most charismatic and talented performers.
If the artists have to start competing by bidding on it, and then paying millions just to be there, it devalues both concepts. Beyonce starts to look like some desperate attention hog in a jumpsuit. Madonna? Prince? Tom Petty? The Who? When was the last time they had a big record? Sad to see them trying to grab publicity by miming their old hits at a football game.
And really, that is the best weapon we musicians have against the whole pay-to-play concept: the idea that it puts lower-quality entertainment on the stage.
I don't want to watch someone who's forked over money to a club owner or promoter so they can play. I'll go to a smaller club in a less-popular area of a city to see people who are playing there because they have a following, or the booking person liked their recording. Screw the Whisky and the Roxy. I don't want to go there and overpay for their weak-ass drinks anyway.