Interestingly enough, I worked at a small startup RJ operator way back when while they were transitioning from 121 on demand charter to 121 scheduled carrier.
They advertised, published schedules, and sold seats to a new city and then realized it had not yet been added to their OpSpecs.

They tried to get a few of us to operate under the 830 public charter rule to go do the flight. I and a several other captains refused the flight.
The pressure finally ended when we said, get us an email from the POI saying this was legal, and we'd go.
we never did those flights until the OpSpecs were updated.

It is different because were selling our own tickets; but I think what JSX is doing is just gamesmanship. This is not the marketing company telling JSX where to fly, it's JSX telling the marketing company where to sell tickets to/from.