I have no idea!
Makes sense. I also don't think google can distinguish between the different sites.. if it works like that for those subdomains, I'd bet it works that way for all the others too.
In reverse... a subdomain is no different than a folder today (in Google anyway) so wordpress could be structured like wordpress.com/my_webspam/ and it only effect those issues on those particular pages.
disavow is exlcusively a Manual Review tool... which suggests Google Webspam Team controls how they devalue your domain... a page, a folder, and subdomain or the complete domain.