This was way, way overdue. For much of its life, MAME was a total mess of license restrictions. Just knowing that a file was in MAME didn't really yield much reliable information about how it was licensed. Some parts of the code were under seriously weird terms, including e.g. an assertion of the "right" to change the license terms retroactively (said license also refers to the code as "copywritten"...).

Kudos to the team for sorting it out, and to all the old contributors who agreed to relicense their code where necessary.