Are flexible natural gas lines difficult to install?

My natural gas line in the basement is the new flexible kind (CSST) I want to tee off this line add a new ventfree fireplace. Why do you have to be licensed to buy/install this stuff?. Obviously it could be dangerous if you're not careful but hey, so are a lot of things. Firearms for example, but those are legal. What's so special about the installing this stuff that makes it a no no for diy'ers?