When I open Tivo desktop and click on the media button, there is a video tab. It says that it supports divx and xvid files, with no mention of needing Plus to use that feature.
Yup, that is a bug. The TiVo website is slightly better, but not much. You only see the list there after drilling down through a link in an area of the page dedicated to the Plus feature. A footnote is needed. You aren't the first to complain about it, so I am fairly confident TiVo knows about it, but you could always suggest it at research.tivo.com/suggestions or in the suggestion avenue forum here.
I don't believe it is scam, just a casualty of their rush to get a vista capable TiVo Desktop out. There were a lot more people complaining about that than this. My guess is they decided to pump out what they had, warts and all, just to give the Vista users a working program. I could be wrong, but that is my humble opinion.
By the way, the plus key pays for the MPEG-LA license for the codecs it unlocks. TiVo most likely doesn't make any money off of it. By using a Plus key, they can offer the program for free to people who don't want or need to do conversions.