1. Optics are not viewed with a helmet mounted NOD, that's what a PEQ/PAQ is for.
2. The reason some of them run their EOs like that is because some guys are running older uppers with fixed carry handles. Even after switching to a flat top upper some guys keep them there because it allows you to pickup the reticle with more speed during presentation.....remember, red dots were originally designed for use on handguns. Picture a cone coming out from the rear of the optic, if your head is in the cone the sooner you can see it, the further away the optic the wider the cone......if that makes sense.
3. You'll see some morons in the regular army do this without realizing that you can't just throw it up on your handguards, these people are stupid and are not to be emulated. You need a floated rail or one that simulates freefloat, KAC/DD ext...the same goes for mounting lasers.