Well not really straight cut as you would think it would be.
This is my opinion on the subject,
In a real world scenario a guy running a pizza parlor wants to have a few arcade games like
Donkey Kong or some other obsolete game that nobody has seen or possibly played before...

...just gets a broken cab and fires it up with MAME and some roms. Happy days.
Unless the guy is a complete moron and puts MAME CAB on the machine, nobody would be the wiser and the guy would crank in some quarters. If the dude puts it on free play more the better as there would be no licensing or maybe just a permit. The mamedevs have no calling back code in MAME nor does it flash MAME IS NOT FOR COMMERCIAL USE every 5 minutes.
Say they guy gets caught with his pants down and the MAME machine is discovered. What then?
A warning? Jail time? Nope. How many local cops you know that would do anything?
Besides its a civil issue.
Mamedev is a collective, each developer that submitted code (and there is quite a few) would have to testify. Then comes the media circus when some company who owns the rights to Donkey Kong or whatever wakes up and sees serious money on the TV. Guess who would be sued for breaking copy protection? Nintendo is notorious for such behavior. Who? The guy in the pizza parlor? Seriously,
who?As mentioned by Saint - the Library of Congress would be the only group that would be in the position to include or protect MAME:
The Copyright Office is conducting this rule-making proceeding mandated by the Digital Millennium Copyright Act, which provides that the Librarian of Congress may exempt certain classes of works from the prohibition against circumvention of technological measures that control access to copyrighted works.
The purpose of this proceeding is to determine whether there are particular classes of works as to which users are, or are likely to be, adversely affected in their ability to make non-infringing uses due to the prohibition on circumvention of access controls.
The Notice of Inquiry in this fourth anti-circumvention rule-making requests written comments from all interested parties, including representatives of copyright owners, educational institutions, libraries and archives, scholars, researchers and members of the public, in order to elicit evidence on whether non-infringing uses of certain classes of works are, or are likely to be, adversely affected by this prohibition on the circumvention of measures that control access to copyrighted works. http://www.copyright.gov/1201/Article I, Section 8, Clause 8 of the United States Constitution, also kicks in here. A real mess if the media got wind of a real scoop and you would get some Michael Moore type reporter locating all these long-gone holders of old arcade technologies (remember this is prime suing time for those with losing holdings in the entertainment business, software houses including) asking for their views. Basically a witch-hunt.
The rest of us who have Mame collections will keep quiet. Maybe we will see a renaissance for all these games, with Mr. Foley riding shotgun.

I personally love what the Mamedevs have done. They are a part of our history, and the revenue generated with all these new ports of old games are a testament to this, something which is not lost on the copyright holders.
I'm not that happy with the cracking of games in the 2000-2009+ era, as the console software houses love kicking these ports out. I have said in the past and I will say it now: Stop bringing out new stuff and fix the old code.
What I am getting at is the stone throwing mentality that some Mamedevs have, considering they are not officially in the Library of Congress (again correct me) negates this fact and have the right to break copy protections.
I live in a dying holiday resort town. I'm used to seeing arcades go bust and I also see some questionable arcade machines in my journeys. Am I responsible to report it? Is there a Mamedev reward hotline?
That's how I see the whole Mame vs the commercial use world. Hypocrisy?
But we are talking about those offenders who operate Mame in a commercial setting as less than 1% - (please correct me) over the millions of Mame users out there all playing robby roto - its a logical conclusion.
Edit:
I read this again and how I see MAME operating this policy of archiving and saving history is to complete work on such game and break the code so roms could not be played. The machine is archived but the operating code is not available to the general public only to the copyright holders or PD.