Seven of the eight players named inactive were already unavailable because of injuries. The eighth, tackle Renardo Foster, wasn't going to be active anyway (teams usually keep seven offensive linemen active for games and the Rams already had seven without Foster).
That allowed the Rams to leave Jackson active without negatively affecting roster flexibility. If he feels good and plays well, great for the Rams. If he cannot contribute much, no big deal. It's not like a third-string tackle, Foster, was going to factor into the plan.
Jackson remains listed as the Rams' starter. He could play a little or he could play a lot, but he did very little during the week or during warm-ups Sunday. And the Rams' decision to name Jackson active meant little by itself for the reasons outlined above.