Hi Craig,
There is no ban on sending them anywhere. I suppose you could get in trouble or at least become a suspect in illegal activities if you were shipping large quantities. It is sort of like papaver somniferum(opium poppy). You can buy seeds for garden use and trade them legally. It takes a lot of plants to make drugs. So if you were buy in massive quantities or growing huge fields of it, the authorities might be suspicious that you weren't just gardening.
By the way, I just happened over here tonight, but there isn't as much traffic as over on the Seed Exchange forum. You probably would of received an answer over there right away.
Remy

The castor bean is a source for ricin, but, assuming that is not the intent of the sender or the receiver, then it is fine as far as shipping them.

This link says weeds, but there are other plants listed on the site. It's helpful, because there are links for each state.

Here is a link that might be useful: State & Federal Noxious Weeds List

Personally, I have shipped castor beans in a envelope. But you have to watch the weight and postage. the post doesn't open your mail, so they won't know either way.