I agree that it smells rotten - that is IF we've got the analysis right. I'm not certain that's the case.
Over the past year, I've come across this inscrutable "doorway page" message a couple times, and it hasn't seemed to apply very well in two of the cases. Especially the "some of your site's pages..." part. My first reaction was that they had a new algorithm and it was misfiring.
In one of the cases where we got that message, there was a penalty but it was removed a month later with NO changes to the site... mostly because we couldn't find anything to change. In another case, there never was any penalty. In yet another case, one that I just heard of (so my analysis isn't as complete), it seemed to me that the penalty they got was related to other factors and not the "doorway page" issue that the message talked about.
There have been a number of confused webmasters posting in Google's Webmaster Help Forum about the doorway page message, but I have not seen a Google rep chime in -- nor I have I seen any consensus in those threads.
So I'm not (yet) willing to take on the conclusion that having sites in more than one niche is a practice that's under the Google gun, and they're using the "doorway page" excuse as their weapon of choice. For me, the jury is still out on that particular leap.