「英語MLの投稿にWeb翻訳を使おうがかまわないのだけど、フリーソフトウェアの翻訳に直接流用してはダメ。少なくともLaunchpadでは修正BSDライセンスなことを求められるので、Web翻訳サービスのライセンスとは相容れないのです。」2017-02-23 12:30:40

Ubuntu Japanese Team がこだわる真の理由はこれでしょう。

I have no problem with BSD myself, but I also uploaded translations from upstream. What do I do?
https://help.launchpad.net/Translations/LicensingFAQ
「As long as the uploads were marked as translations that were published
elsewhere, they fall under a separate copyright regime: those imports will
retain their original copyright license. The BSD licence only applies to
translations that are (as far as the system knows) original to Launchpad.」

　↑　この規約と整合性を取るのが目的だと考えられます。
彼等の論理で行けば、もし整合性が無いとライセンスに違反してしまいます。