The reason for that is b/c anytime a split of some sort (R/S or F/S) takes place, the company goes through a new CUSIP # (I Believe) and all the old shares have to transfer to the New symbol and/or CUSIP number - thus the hold on being able to trade any of your previous holdings.
You are allowed to trade any NEW shares you buy in that company under the new symbol and/or CUSIP of course :)