They deliberately misinterpret that quote. The "well-regulated militia" refers to the federal government's standing army, and that while having that is important, people should still be able to arm and defend themselves from any danger whether it comes from other people or the government itself. But they lie and insist that the 2A only refers to an army or getting permission to have a militia which couldn't be further from the truth.
Right, I'm going to be that guy. Not because I disagree, but because word autisms are occasionally my thing.
Regulated today means, well, something that is regulated; something that the controlling authority has created a set of regulations to define and proscribe. Something that behaves according to those regulations. When the constitution was written, regulated had a subtle different meaning, as something that is regular or consistent in its constitution and make-up. If the second amendment were being written today, quite apart from getting the clauses separated correctly, the framers might have phrased it something like this:
"In consideration of the fact that a suitably supplied and armed militia is necessary to the security of the state, the right of the people to keep and bear arms shall not be infringed."
Layman's approximation, of course, but this is sort of what they had in mind. The framers didn't want a permanent standing army, but they wanted the ability to quickly form a properly equipped and armed force when necessary, which means that in order for the militia to be well-regulated, all the weapons necessary for that state to exist must be available to the people. By the original intent of the constitution, a free citizen of the united states has the right to own a B2 bomber.