I never have seen what's so hard to understand about it. A well trained Militia is needed to protect and secure the Freedom of a State(remember, the Founding Fathers wanted States to be the "power" not the Federal government) from all enemies so they need the freedom to keep and carry firearms that are suitable for military use.
That's why I get so upset over all these folks asking "Why do you NEED a semi auto AR-15?", because they're suitable tools for military use, that's why. Heck, if we could get the '34 NFA thrown out we could have even more suitable tools in the forum of REAL full auto assault weapons, the way the Founders intended in the first place.