Indycar is broadcast on Fox starting last year, and Fox bought 1/3 of Penske Entertainment (i.e. the Indycar series, the Indy 500, and IMS) over the summer. While ratings outside the 500 are generally not great, they have been on an upward trend, and open wheel formula cars have been "hot" culturally in recent years- I guess some figure there isn't enough room on the F1 bandwagon for everyone and series like Indycar are in a good position to pick up the stragglers. The amount paid to charter teams isn't going to pay for the season on its own, but it does help.
That being said, please please NHRA/IHRA/anyone else stay away from anything even vaguely like charters. They are awful and completely against the spirit of motorsports. They're terrible in F1, terrible in NASCAR, and terrible in indycar. The most fundamental tenet of racing is that anyone can show up with a legal car and take their shot. Be fast or go home, it doesn't matter who you are. Just like "playoffs," franchises are an aspect of stick and ball sports that has no place in racing.