Aren't you supposed to use the run to set up the pass? Run Mason for 3 yards, and then you set yourself up for a 3rd and goal from the 3.
You're assuming a run would have been successful, but I think perhaps the earlier 3rd-and-1 where they got stuffed weighed on Fisher's mind? Also, they (hopefully) had studied the data on SD's goal line defense and felt they were more vulnerable to the pass?
With the way the game was going (with the refs and the D just gave up an 11-play 90 yard TD drive or something?) I think Fisher felt overtime was a losing proposition and it was no time to play it safe. It was time to win now. Yes, running it could be the way to win, too, but he obviously calculated running it would get stuffed, so decided to take the 2 shots passing into the end zone. Passing on that 2nd down was an unexpected choice and could have caught SD off guard.
I myself would have run it, but I totally understand the choice to pass there. Even if I personally disagree, it's simply not a criminal act by Fisher, it's just a choice that didn't work.
What is a crime is the empty backfield at your opponent's goal line. Sure some teams do it -- when they have Peyton or Brady as their QB. The Rams don't and need to get real that our QB needs that run threat helper. Just a terrible play call by Schotty. Just like the pulling lineman on 3rd-and-short is idiotic, run the goddamn QB sneak there. Sigh, sorry I digress.
My final thought that's bugging me is the wild inconsistency with the Fisher regime. Once upon a time we massively overspent for "playmakers" like Tavon and Cook. Then we didn't know how to use them. Then we declared a "do over" and that the Rams identity is a smash mouth run game with preaching winning 13-10. And still the Rams have no lead blocking FB -- not even when our lone lead blocker, Harkey, is injured.
Well, what is our identity? Are we a conservative smash mouth team? If so, we shouldn't pull linemen on 3rd-and-1, but line up jumbo and ram it down your throat. If so, we don't go empty backfield on the 6 yard line, but jumbo and ram it down your throat. A smash mouth team stays with the more conservative choice and runs with an air of inevitability, crushing the spirit of the opposing defense. And I won't go off on the tangent of how too much blitzing on the other side of the ball plays into throwing off the conservative team identity... Overall I just don't get Fisher -- too much offensive cutesy and wild lack of focus on defense for his supposed preferred conservative identity. Not necessarily a criticism of Fisher since being unpredictable can be an asset, but just rambling that I still don't know what the team's identity is!