I guess they might be looking at the generic Chromium user agent string, which does not get used a lot on Windows and also infer from that that you might be a bot, even though they do not list that as a reason.
If you switch the user agent string to indicate Linux, the users of which more commonly use Chromium (I believe, given the various distributions), it might be more well-received.
Another reason could be a bug/feature that is currently in development. Does Chrome canary exhibit the same issue? If not, then I would probably bet on the above.