The throttle can also stick if the cables are too tight. There should be 1-2mm (about 1/16") play in the throttle. There's tension adjusters on each cable.
It can also get sticky if someone's used the wrong lubricant on the cables (eg: chain lube). If you're going to use any lube at all it should be a silicon based cable lube, however you shouldn't ever need to lube these cables. If someone's sprayed something down them you'll probably need new cables.
