If the throttle on your motorcycle doesn't snap shut automatic when you take your hand off the grip it's dangerous. In the event of an accident, the throttle should close and the motorcycle should stall or idle gently. The chances are that the rear wheel will cause much damage as you could end up pinned under a motorcycle with a racing wheel and chain grabbing at you. It might rev like mad and kill the engine due to oil starvation, or it could even manage to bounce up and keep going like ghost rider.
Like a lot of constantly used parts on your motorcycle, the throttle needs regular maintenance. You might want to check the throttle is moving freely on the handlebar, so twisting freely. That might be caused by the bar-end catching the twist grip; the twist grip being dry or caked-up and dragging on the handlebar; or it could be the throttle cable is clogged up and dragging in its sleeve.
Unscrew the handlebar-end weight and check if it's catching on the twist grip. If it is, the solution might simple be to move the twist grip assembly along a fraction.
The twist grip assembly itself is a little fiddly to understand but not a complicated job to dismantle, check and put back together. Most motorcycles have two bolts clamping the assembly housing onto the clip-on. Inside there'll usually be a throttle cable or two with a nipple on the end slotted into a hole in the twist grip head. It's easy to take that out but, before you do, remember which way around they go for re-assembly.
With the bar-end weight removed, the twist grip housing off and the cables unattached, you should be able to slide the twist grip off the clip-on.
Give it a good clean, either with a clean cloth or, if it's particularly dried and manky, I'd give it a dose penetrating spray, then get the cloth in there. The same goes for the inside of the twist grip and the clip-on, though be careful not to get penetrating spray between the actual grip and twist grip. Give it all a thorough wipe clean and then, personally, I apply a very thin layer of 3-in-1 oil to the clip-on to smooth the action of the twist grip on the handlebar.
Use a cable lubber on the cables themselves is good maintenance but don't spray too much in. If you do, then use a cloth to soak what runs out the other end. A graphite penetrating spray like Silkolene based spray is good, as it won't react with the nylon inner layer of the cable sheath which can cause problems. New cables aren't expensive – but would be more hassle than just lubricating the existing ones as you'll have to detach both ends and route them through.