I see two issues with this.
First of all, this could be an issue in Canada. If we can import the upper reciever and the rest of the gun (unrestricted parts, correct?), then us Canadians (who can't normally get access to "assault rifles") would be able to create one without a license to do so.
Second of all, this could be an issue if using a WE reciever would enable full auto (although we haven't seen any evidence of this yet).
Anyway, we all knew this was coming.