Everything I’ve read on the subject says that the non-elimination legs are determined in advance. I think Phil even used to announce it specifically within the show. (It seems the in-race descriptions have been a bit streamlined this season.)
I don’t know if there’s any sort of independent monitoring of the Race and all its rules, though.
My understanding is that determining which are the NELs based on who’s eliminated is illegal, as it’s essentially fixing a game show.
The number of non-elimination legs is announced during the first show, but whether or not it is predetermined which actual legs those will be, is another matter.
Phil used to say that “This is the first of X predetermined non-elimination rounds*” In recent years he’s stopped saying that. And some of the non-elims have gotten sketchier.
Remember, it’s not illegal if the contestants agree to it in advance. If their paperwork says “We’ll do non-elims based on weather, production needs, etc at our discretion”, they’re fine.
*There were a few obvious exceptions. Whichever race had Lori and Bolo, for example, there were Shenanigans, but that was because some war or something broke out IIRC–it was discussed after the fact by some of the contestants.
Another “odd” occurrence was the time in TAR All-Stars when (due to their really, bizarrely weird airport skillz) psychos Myrna and Charla got something like 24 hours ahead of everyone–including the production team apparently. Rather than just say “Y’know, this would ruin the race, so we’re just gonna let them have a 4 hour lead”, they claimed that “bad weather” delayed Myrna and Schmirna for a huge amount of time…and the Amazing Cameraman focused on the perfectly clear blue sky. 
I’d be interested to see if the Goddamned Luser Faux-Hippies season had Phil using the “Predetermined non-elimination rounds” line, because either those assholes were the luckiest bastards alive or someone on the show was willing to go to some pretty intense extremes (beyond what they’d already done for the ‘hippies’) to insure their victory.
Then there was the year that we suspected that a flight, containing as I recall only one team, was delayed by the producers so that a couple of other teams could get on board. This was at the very end, with only 3 teams, so there would have basically been no finale show with one team arriving hours ahead of the other two teams.
That’s assuming TAR is a legit game show. I rather doubt it is, much like Big Brother, which the FCC considers a ‘‘for entertainment only’’ program like professional wrestling. Same with Survivor, no doubt.
Put it this way: what possible incentive does CBS have to conform to game show guidelines with any of their big three reality competitions? How would that help the producers in any way? The upside of not having to worry about Quiz Show-type controversy has to outweight any advantage, and frankly I don’t see a single advantage anyway. Nobody watches Big Brother/Survivor/TAR for the integrity of the competition.
Between the three, I’d rank TAR as having the least competitive integrity, with Survivor having the most. Not that that’s saying much for Survivor.
It would explain why the Speed Bump has been a lame task that could be added to any stage of the race - “go stand by the tree and count to ten” *.
I think a Speed Bump should be comparable to a detour task, with a mitigating factor being that the stage is a double long killer fatigue stage. The last place team has a serious extra task, but everyone has a real long hard stage.
Best. Detour. Ever.