I feel that way, too. I mean, there's almost very little point to assigning him to one show or the other seeing as how he only works a few dates a year anyway, and since Brock already has a special deal in place (both in real life and on the show) where he gets to come and go as he pleases instead of being permitted to work as many dates as the rest of the herd, it's logical that they'll make a small exception with him in regards to the draft as well.
In regards to other top stars crossing over from time to time, yes and no. Of course there will be inter-promotional matches from time to time, but I would expect stars to stay restricted to whichever brand they're drafted on at least 99.9% of the time. Otherwise, it just makes the whole idea appear meretricious.