IMO, the word "regulated" evolved as the government used it to sell the public on prohibitions and restrictions on our right. It just sounds better to say regulation than restriction or prohibition.
It doesn't make sense that the Congress could protect a right, and give itself power to restrict a right in the same sentence.
And, again, "well-regulated" is already moot at this point. The only reason it comes up in debate is because the anti's are still using it.