My reasoning is that a decent cop would think that anyone who bothered to register an SBR with the feds probably has his/her ducks in a row and leave them alone at that point, assuming that it even got to that point, which I find to be highly doubtful. The overwhelming majority of decent cops in this state stop caring about what you have once you produce a license. (and sometimes, not even that much)
If your asking about the reasoning on "it going both ways" read EasyD's NFA faq in the NFA subforum, both arguments are presented in there, in detail.
-Mike