This is true, Farah does appear to be unable or unwilling to push the pace in most races. I used to think this was because he couldn't but after the 2 mile world best he ran solo I think it is more because he doesn't want or need to.
He doesn't gain much by winning a diamond league in say 12:45. All he does is show the competition what his absolute top ability is. For all these guys know, Farah could be in WR shape, so it isn't worth running a hard championship final for fear of losing out on silver also. This way they better play into his hands in the big finals. Then again, he may just find it difficult to run from the front, not everyone can. Think Amos.
Athletics is strongly a mental game also.
Farah doesn't make the races slow, he is one man in a field of 10+. The other runners choose to make it slow. Farah runs just fast enough to win, which at the end of the day is the game. You see runners in shorter events easing down once the win is secured all the time (Bolt, anyone?). Winning and medals is what matters really.
I strongly believe if in Rio the other guys take it out in sub 12:50 pace then Farah would set a new PB to win; just keep holding on in 2nd/3rd then kick hard in the last 100m to take the victory.
I also think this would be true in a DL. Farah hates to lose. So if anyone wants to see what Farah can really run in a 5k then they need to get on at the other guys to make another Paris 2012!