Source?

Fair use is whatever a judge and/or a jury says it is.

Fair use isn't a feature of copyright in every juristiction, which could make this a less than useful idea trying to create a global corpus of speech data.

Which, it must be said, isn't always as bullet-proof as it could be. There's a not insignificant amount of transcription (or pronunciation) errors in those datasets and Mozilla might want to find ways to increase the quality of already-released data over time.