Still... There's this "well organized" bit. Which means organization and structure, following orders and such. Where the firearms are kept is not specified, so it's secondary. Could be at home. Could be at militias armory. Could be that guns are kept at home but ammunition is stored in the armory like the Swiss do.

My reading is that the intention was for the states to have right to keep Swiss style volunteer armies, but what was dangerous was the "well organized militia" bit so it's not implemented. Instead guns and ammunition are allowed to be freely distributed to prevent the organization and exacerbate chaos between the people in the event of uprising against the government. If the anarchy comes, because of the prevalence of guns and lack of organization local warlords will quickly arise and mostly kill each other in struggle for power. Then the oppressive government comes in and just sweeps the thinned herd.