Actually i have proof sniper rifles are in the game, there is one shown in the cinematic trailer and one is shown in the Security character customization gallery: 3rd row down and 16 to the right.
The cinematic trailer has a lot of inconsistencies, showing things that aren't in the actual game (like molotovs, dual wielding pistols, wall flips, and the way they depict sniper rifles)
As for seeing them in the character customization - it has been mentioned numerous times that the "sniper rifles" in Brink, look like traditional sniper rifles, but do not operate the way you'd expect a sniper rifle to behave in a game. They aren't even called sniper rifles in Brink, they are known as short or light rifles.