Personally, I prefer to use the Barrett or AS50 as my sniper rifle of choice. The one-shot kills and semi-auto performance suit me well. I believe the AS50 is quickly becoming my rifle of choice.
But to stay on topic, using a semi-auto assault rifle in a sniping capacity isn't so far-fetched. True, it is classified as an assualt rifle, but with its specs can you really blame somebody for using it in that role?
I don't know about the thermal, but using a scope on the MK14 does seem to legitimize it as a gun used for sniping.
In reality, you can snipe with pretty much any weapon, though of course its preferable to use a more high powered weapon for long-range engagements.
The bigger issue here is that there really is no distinction anymore between weapon classes on this game. When an SMG like the MP7 is as deadly accurate from long range as a sniper, why would anyone use the sniper? When you get the same range from the MK14 as you do from a Sniper, with a better fire rate and less recoil... why wouldn't you use it over the sniper rifle? If they had done a better job delineating the weapon classes, this wouldn't be so much of an issue. Snipers should have an advantage at range with a greater disadvantage in close quarters environments. AR classes should have Decent range and be effective in CQB as well. SMGs should be dominant in CQB but lose effectiveness over range. The way it stands... you can pretty much use any weapon from any range in this game.