"Public Interest" channels pay for retransmission ... the rates are based on the cost of the transponder space. Satellite carriers must make a certain amount of space available at each satellite location for PI channels. If there are not enough channels willing to pay for the space satellite carriers don't have to meet the quota.
The channels must be non-commercial (no advertising except what is allowed on a non-commercial station) and there can only be one from each provider (unless there are not enough providers to buy all available slots).
DirecTV has it right ... they offer channel space for sale to broadcasters such as INSP. INSP has it right that their programming is free. What INSP has failed to mention (or contradict) is that they paid for that satellite space. They think DirecTV should carry them for free.
INSP needs to pay for the satellite space just like they would pay for any other distribution method.
INSP carries plenty of commercials so there must be some sort of confusion here.