They do not want Americans to have firearms similar to those that they jealously horde for their own use.
Check it out:

In the May issue of Townhall Magazine, where this article originally appeared, Bob Owens explains how the ATF is stretching the definition of “firearm” to seize data on law-abiding Americans.

It has always been legal to build your own firearms in America, and has been since well before the United States existed as a country. No forms must be filled out for the federal government and no permission needs to be given. It is your inalienable right.

If there is one thing that we’ve learned from President Obama’s “Bureau of Alcohol, Tobacco, and Mexican Firearms Delivery” it’s that they do not want Americans to have firearms similar to those that they jealously horde for their own use.