How is Gun Registration and Owner Licensing Even Legal?

Folks, this is something I have been wondering for a long time and it is really simple:

Why has no entity or citizen challenged the legality of firearm registration and/or owner licensing in any State that requires it under 18 USC 926(a)?

This section of the law reads as follows:

(a) The Attorney General may prescribe only such rules and regulations as are necessary to carry out the provisions of this chapter, including—

(1) regulations providing that a person licensed under this chapter, when dealing with another person so licensed, shall provide such other licensed person a certified copy of this license;

(2) regulations providing for the issuance, at a reasonable cost, to a person licensed under this chapter, of certified copies of his license for use as provided under regulations issued under paragraph (1) of this subsection; and

(3) regulations providing for effective receipt and secure storage of firearms relinquished by or seized from persons described in subsection (d)(8) or (g)(8) of section 922.

No such rule or regulation prescribed after the date of the enactment of the Firearms Owners’ Protection Act may require that records required to be maintained under this chapter or any portion of the contents of such records, be recorded at or transferred to a facility owned, managed, or controlled by the United States or any State or any political subdivision thereof, nor that any system of registration of firearms, firearms owners, or firearms transactions or dispositions be established.

Nothing in this section expands or restricts the Secretary’s ^[1] authority to inquire into the disposition of any firearm in the course of a criminal investigation.

(b) The Attorney General shall give not less than ninety days public notice, and shall afford interested parties opportunity for hearing, before prescribing such rules and regulations.

(c) The Attorney General shall not prescribe rules or regulations that require purchasers of black powder under the exemption provided in section 845 (a)(5) of this title to complete affidavits or forms attesting to that exemption.

Emphasis is mine.

Can anyone tell me how any State-level system of registration of guns for any purpose is legal such as many States for handguns, California and Maryland for "assault weapons" (among others) or owner licensing such as in Illinois and Massachusetts given this statute?

I don't care what they call it. Sales records, safety inspection (Michigan's excuse) or an up-and-up registration database, by the reading of this section, are all illegal.

FOPA'86 was passed for the most part to protect gun owners from overreach by State governments against US citizens.

So can anyone tell me why any of these systems of registration exist when the law specifically says the States or any political subdivision of the US government (which would include cities) can't create such things?

Couldn't someone tell a State to pound sand with regard to getting a firearm license or be denied registration, be denied such as what occurred in DC which lead to Heller, and then challenge the legality of such requirements as illegal under 18 USC 926(a)?

Why do these systems even exist given this statute? Is there something I'm missing?

Registration of guns and owners is a big issue with me. I plan to explore it a little more in depth soon but if anyone would like to chime in with their thoughts, I'd love to hear it. Especially if you're an attorney specializing in 2nd Amendment cases.