Sometimes you can get a better field of view with RDS further to the rear, since less of the non optic part is covering your (primary eye) vision....better dynamic look up characteristics etc. I have found with RDS, further back, the less perturbation of it from movement given the lower moment arm in play too (esp good for dynamic situations)...given 1x magnification has unlimited eye relief, thus can technically put it anywhere as long as its not too close (when use of two eyes as should be done is compromised). Some prefer further forward on the rail, it comes down to preference I think...given traditional thinking of sight radius and it could have placebo effect (because in reality there is no sight radius for RDS given no conventional dual reference)....and of course there is small interplay of having the body of optic cover less FOV of the second eye, further out it is....and there will be a sweet spot for most people (w.r.t FOV of both eyes) depending on the model of RDS, personal preference etc. So everyone can find what theirs is by testing many configs.
Maybe
@jhungary can add something to this from his experience.
Why is the RDS positioned so far to the rear?
There are many factor affecting how people use their reflex. When it come down to the final (ABSOLUTE) decision, it's based on the comfortableness of the user, so the end question is, Are you comfortable with the position. Which if your answer is yes, then that's the position for you, I have seen both case, people who advocate forward mount and people who advocate rear mount. (Nobody seems to do mid-mount tho....maybe I did not ask enough people??)
Anyway. it boils down to sizes, accuracy, weight and the need for red dot sight.
Red Dot sight actually emulate the foresight (not back iron sight) of a firearms, it replace the front tritium (did I spell it right??) dot and replace it with a red dot that you use for quick reference. The problem for people who pull the sight closer to the eyes is that if your sight is not absolutely zeroed, your sight will not be as accurate as forward mount RDS, because the length between your eye and the dot is closer (actually extremely close) one slight different between the target and the dot (which is far because you mount it closer to your eye) would magnified the gap, which translate to the sight will not be accurate, unless it is perfectly zeroed. This would affect mostly Rifle because of the long barrel (usually 14 inch plus) but not much on SMG/Shotgun because of the shorter barrel.
Putting RDS forward also help with weight distribution, especially with full size RDS (like aimpoint), because rifle or weapon are usually tail heavy (with the exception of Bullpup of course), added a full size RDS would counter balance the back, and you will get more stability.
The people who purported to mount it on the rear is those who uses RDS (mostly mini RDS) on a SMG, Shotgun and/or Pistol. Putting it way back means you can see thru the scope and the rest of the scope did not hinder your field of vision, unlike mounting it forward when the scope itself obstruct part of your view, like
@Nilgiri said. It also provide a quick reference to your target so you can switch around with your aiming eye to aim and your non-aiming eyes for situational awareness. But one thing I need to say is that mounting it forward give you more field of vision in general, as oppose to mounting it backward.
On the shooting style, it's quite depend on individual, some like to look thru the scope (especially transitioning between RDS and magnified sight), by putting it backward it work like a mag scope, some like to look over them (by putting it forward, you just glance at it when you need to) This mean if I put my red dot back and closer to my eyes, I am "Always Aiming" but when I put my RDS forward, that mean I am "Always looking" Depending on situation, in CQB, you should not be always aiming because you will need a great deal of situational awareness to fight thru close quarter, but in open field, you can stick with always aiming.
My personal preference is putting RDS forward, I am adapted to both school of regime, most people who used magnified sight would put RDS backward, that just how they are used to, so, as I said, when it came down to the final question, it's up to that individual.
I hope this is detail enough and clear enough response for the question. If you have other follow up question, please do not hesitate to ask, because it's very hard not to quote specific term (like MOA or Reflex Cap) or term that I need to explain...
