It's a problem because they aren't doing everything they can to get him ready to start, just as you said. Keenum is what he is and should be able to get by with a reduced number of snaps. Goff should be getting the majority of reps so he can get READY. If he isn't used to taking snaps under center or calling plays in the huddle or making line calls or calling audibles or whatever, repetition is the only thing that's going to get him ready.
If this was a 2-14 team that was a mess and there were many pieces not in place, fine. Let him sit so he doesn't get David Carr syndrome. But this is a team that's supposed to be a QB away from the playoffs. Why restrict everything he's doing? Less practice reps? Less playing time? How is that supposed to expedite his learning and readiness?
There are too many examples of rookie QB's coming in and, at the very least, function in recent years. It's perfectly doable. Unless you're Jeff Fisher. I mean, is Goff so far back from playing at Cal that he really shouldn't be starting for the, until further notice, pathetic Rams offense? How could he be any worse than dead last in the league?
And there's never going to be a perfect time to start him. He isn't going to sit on the bench for 8 weeks and then, boom! He's ready to be an NFL starter. Put him in with the starters, let him play 3 quarters and run the real offense next week and get the ball rolling.