You're mixing up capability with likelihood. Total risk is the product of the two. The U.S. has had nuclear-capable ICBMs for over 50 years now, but has never used them. So while it has had the capability for a long time, the proven likelihood that it'll use them is very low, even when it's been provoked. The reason people (not just the U.S.) is concerned about North Korea's capability is because its leadership is extremely erratic and unpredictable, so the likelihood it would actually use ICBMs is a lot higher than existing nuclear powers'.
On the contrary, NK has had nuclear weapons for quite a while and has never used them beyond testing. As with any mutual-assured destruction weapon, showing a capability for something does not indicate anything about willingness to use them at any time except a doomsday scenario.
Depending on the success of this test, and certainly prior to this point, NK only had MAD capability against its immediate neighbors, China, South Korea, and Japan. The only deterrents they had against US invasion were indirect, through threats on US allies. A working ICBM gives them a better ability to deter the type of regime change we pulled in Iraq.