Well...in the IG it's typically the veteran and ratling units that get heavy access to sniper rifles. Both of those units have superior BS compared to regular guardsmen...and marines give their sniper rifles to raw recruits who aren't even full space marines yet so it's hardly surprising that they do not have superior bs compared to other marines. Every space marine started life as a sniper, so of course a fully fledged mrine is going to have the same or better shooting ability.
Plus the lethal accuracy of snipers is covered by the sniper weapon rule. A sniper rifle is so nasty because its user usually gets you in the vitals...if you're aiming for the vitals you're aiming at a smaller target and thus it takes more training to hit, giving them an overall comparable BS Someone with a sniper rifle can headshot you with the same ease as someone with a lasgun and the same bs trying to hit your chest...and conversely will miss just as often.
He can also do it from further away. Being able to hit a target just as well but from half-again the distance is a measure of accuracy as well
Also, that has fuck all to do with the fact that weapons are weapons and wargear that is a weapon is a weapon.