Wouldn't it be possible to include a license with the release stating that the assets are only for use with the game? You can get at most game assets once you buy a game anyway, it isn't that hard to extract them but you're still not allowed to reuse them without permission
The Unity Asset Store license agreement has this:
"Licensor grants to the END-USER a non-exclusive, worldwide, and perpetu- al license to the Asset to integrate Assets only as incorporated and embed- ded components of electronic games and interactive media and distribute such electronic game and interactive media. END-USER may otherwise not reproduce, distribute, sublicense, rent, lease or lend the Assets."
That means that assets cannot be distributed as image files, even with a clause they can only be used with the supplied source code. When art is commissioned directly there is usually a similar clause. Open sourcing a tanking project will almost invariably require stripping all 3rd party assets from the project before releasing. Note that this clause also includes various libraries and source code files.
From the screenshots and videos I've seen, that means that in practice open sourcing TWS would not be legal without removing large amounts of content. That's not a particular pleasant task to spend a lot of time on, for a developer who is probably already quite down for obvious reasons.