[Is MAME and all its roms now legal? (full set is on Archive.org)]

Well, unfortunately I don't think this makes it legal.

 

According to the Archive.org's terms of use: http://archive.org/about/terms.php

 

The pertinent paragraph being:

Some of the content available through the Archive may be governed by local, national, and/or international laws and regulations, and your use of such content is solely at your own risk. You agree to abide by all applicable laws and regulations, including intellectual property laws, in connection with your use of the Archive. In particular, you certify that your use of any part of the Archive's Collections will be noncommercial and will be limited to noninfringing or fair use under copyright law. In using the Archive's site, Collections, and/or services, you further agree (a) not to violate anyone's rights of privacy, ( not to act in any way that might give rise to civil or criminal liability, © not to use or attempt to use another person's password, (d) not to collect or store personal data about anyone, (e) not to infringe any copyright, trademark, patent, or other proprietary rights of any person, (f) not to transmit or facilitate the transmission of unsolicited email ("spam"), (g) not to harass, threaten, or otherwise annoy anyone, and (h) not to act in any way that might be harmful to minors, including, without limitation, transmitting or facilitating the transmission of child pornography, which is prohibited by federal law and may be reported to the authorities should it be discovered by the Archive.