DIY gas line plumbing

I had a new gas line branch only added at the meter for a new second floor addition and kitchen remodel. I need to continue the rough work to reach the various appliances. Flexible SS line was used and I wish to use it as well. I notice most places will only sell flexible SS line and fittings to licensed installers. Is it really taboo, or even against code to DIY. Seems like pretty simple connections. The city (Seattle) didn't seem to balk at selling me a permit to do gas line work.

Wally

Explosions like the ones in the movies are one reason that the makers of the pipe, and the code enforcers like to see a licensed installer.

I fnd that the old style threaded pipe tends to seal better.