they are much slimmer than other locks, except for the cheap plastic ones with the 2 pronged key.
The "cheap plastic ones with the 2 prong key" are perfectly legal trigger locks. If you lose the key, you can open it with a needle nose plyers or multitool. (Every other person at the range has a multitool.)
In addition these locks can actually take longer to get off than a key or combination lock, thus providing marginally more security. See the following:
http://www.youtube.com/watch?v=P397UsoyNBc
I never said they were illegal, I was just accurately describing them, they are plastic and they are cheap, $10 for a dozen and they use a 2 prong key. I don't have a problem with them other that they are a pain in the ass, I personally prefer combination locks so I can set them all to the same code.