The mame licence only specifies that mame cannot be distributed with roms included (i.e., one package). Mame itself has always been legal, emulators are legal.
Some countries may find what archive.org are doing to be illegal but there's nothing they can do about it.
When did it start? It came to mamedev's attention only about a week ago, but no rules or conditions are being broken.
The only question remains is if you yourself have any problem with it. If so, don't go there.