My guess would be since they are a shelfish derivitive, there is some level of concern. I believe this has been addressed and that they are safe to use but I am not sure.
Rocky, you are correct in that Chitosan is made from shellfish... but the important thing to know is that the clearing agent is made from the chitin (the soft or hard shell) portion of the animal and NOT the protein portion of the shellfish. The protein part of the shellfish is what folks are allergic to.
I was once told by a kit manufacturer that Chitosan IS safe, even for those who are allergic to shellfish. It is a settling component that is used in many water treatment plants around the world. If it were not safe, they couldn't use it in that sort of environment.
So the question might be: Why isn't it approved by the TTB for use in commercial wines? For two reasons. The powerful wine lobby in Washington sees wine kits as a potential threat to their industry.* Also, the manufacturers of Chitosan have not sought to fight this lobbying effort by paying beaucoup dollars themselves for testing, proof, and counter-lobbying. What's the point?
*the funny thing is (and I have read articles about this that cite studies done) that people who make homemade wine never stop buying commercial wine. In fact, they buy MORE than before because they are 1) more interested in wine in general, and are prone to experiment / taste once they know more about wine, and 2) they are always on the lookout to compare their wines against store bought wines, or 3) need to buy wines to top off their losses due to excess ullage in the carboy.
I can honestly say that before I started making homemade wine almost 20 years ago, I almost never bought wine in a restaurant. Now, having a glass of homemade with my supper is so ingrained that NOW I typically buy a glass or two when I'm out having dinner at my favorite restaurants. How's that for increased consumption?
Plus, now I'm always on the lookout for new wines to try and wineries to visit. Do I sound like a threat to the wine industry? I think not.
Sorry, I digress. Hope that helps explain your question about Chitosan. Cheers!