I agree with BigRed.
You got to remember that years ago DHTC was the only folks in Canada with the EO, and had old carry over C8's from before the flattops.
The RAS while quite solid is still not the upper receiver in terms of rigidity, so your EO or AP will take more G load from firing on the RAS than it would mounted of the Upper itself.
WRT to making room for NVG's -- these days folks that the Hill go after often have NV of their own, so running with a MFAL blazing is not always a good idea, so using the optic in conjunction with the Dual Tubes is a good way to remain passive. This is the reason many companies are making high optics mounts so as to give the user the ability to function passively using their nods and day optic (using NV setting).
The Assaulters at DHTC shoot a ton of rounds, and among the best in the world at their trade, however I will say that few are gun guys, and since the universally stupid and restrictive Canadian laws do not allow for the use of similar weapons outside of work in competition, certain things do not progress as fast as down here. Kyle Lamb, Larry Vickers, Jim Smith, Pat McNammara etc did not get to be GREAT shooters for being the CAG for so long, they will tell you it was also their competition shooting on the civilian side that drove them to improvise techniques and ideas, that rounded them out - as they had a solid operational side to know what was fast for gun games, and what translated over to shooting people fast and accurately. So my guess is that many have simply keep the EO up on the RAS as that is what was done in the past with the fixed CH, as well when you can get new EO's quickly and easily, if your EO dies more often on the RAS, who cares? Just my guess.
However I will say if you look back years you will see Aussie SASR, 22SAS, CAG and DevGru using the CH, and the EO/AP in front, these days no so much. You will see it on MOST Maritime teams due to the whole Electro-optics and water/ships hulls are not as 100% as some like. However even still despite height over bore issues, you will see a lot of team that use smoke/gas a lot on HR entries will run an EO or AP on a CH to get a better cheekweld when using a gasmask.
Several of those pictures are from 2002-2004, a whole lot can happen in 10 years.
Despite the fact I think the receiver is a better place at the end of the day, its the guy going thru the door who makes the call, and its his ass on the line, so I won't second guess any of them one bit. If he is comfortable with it there and has the muscle memory for it there - then no one can call him wrong.