Man, I hope that person is ok.
That shouldn't happen IF the throttlemester is adjust properly, which it does need to be, by making sure the tension on the twist grip is enough to hold it in place, but not too tight that you can't turn it forward with it in cruise position. (don't ride the bike down the rode until you have tested it without the engine cranked) - the tension is set by how far inward it is mounted against the twist grip, as it is turned it rotates in toward the grip, putting pressure against it. If it was set completely against the grip while turned all of the way out, I could see how that could get so tight you couldn't cancel it in a quick decision situation.
I'm not a salesman for them. mine was on the bike when I bought it. That being said, I rarely use it because I don't really get tired enough to need it. even the 800 miles to Leakey & 800 back I don't even remember using it.
I used it a little bit when I first got the bike until I got used to riding it.