Yesterday evening, Oklahoma announced that their home game vs. Army on 9/22 would air on pay-per-view, both on linear TV and online. It got quite a bit of backlash & derision from fans of schools not named Oklahoma in my mentions. Please don't construe this as an attempt to defend the practice of games airing on pay-per-view, though it happens a lot more than you think, though not at the price point of a single game for nearly $60. Conference USA was behind a paywall for many games until their recent switch to ESPN+, which itself is a monthly pay service. Some FCS schools still charge for webcasts on a per game basis.
Here are the things that ran through my head last night in no particular order of importance.
1) The (in)stability of the Big 12 around 2011 created the situation of allowing for each school to monetize a package of athletic events, which includes one football game per year, and there's a personal responsibility for each school undergoing a contract negotiation for these rights to be diligent about where they want these games to air. Oklahoma gets $7 million per year from these rights and to my knowledge, only Texas' payout from the Longhorn Network is more than this. From a business standpoint, FOX has to figure out how to make money from holding those rights too and they did not elect to have the full time linear TV channel that the Longhorns do. FOX holds the cards here. If Oklahoma didn't want their football game distributed via pay-per-view, they could have forced the issue. FOX could have pushed back on monetary terms.
2) Pay-per-view is a demand driven business and Oklahoma football is no stranger to the medium. Since 2006, only the 2010 season did not have a pay-per-view game. I can't say its an accepted practice, but both sides seem to feel there's enough support from a sales & viewer perspective to keep doing it. It seems to be one that has more of a demand than the other schools that FOX has Big 12 deals with, as those schools usually have their games on FSN.
3) FOX can game the system a bit here. They are the ones who select the member retained games for each school. They also are the ones who would receive a game if the school declines to monetize it. Gaming the system in this case could also be due to scheduling considerations as it looks like FOX will have more available college football windows on 9/1, the date of FAU at Oklahoma, compared to 9/22.
4) Big 12 pay-per-view has always been a unique animal in that the option is supported at a national level, compared to the hybrid nature of how SEC & ACC pay-per-view used to operate, where the pay-per-view territory would be limited to specific states and/or TV markets, then viewable on ESPN3 and ESPN GamePlan. From that perspective, buys are going to come from fans of the road team too and with this year's opponent being Army, there should be might be more interest in purchases of the game from Armed Forces organizations inside the continental 48 (AFN is showing it free of charge outside this area).
Some credit goes to Oklahoma for scheduling well enough to monetize a game that involves a FBS opponent more often than not. The last time Oklahoma's pay-per-view game was a FCS opponent was 2012 vs. Florida A&M.