I'm aware of this rule and agree that the committee boat and the pin end mark are obstructions but as far as the imaginary start finish line between them I find hard to believe can be defined as an "obstruction".
This is a good question as I wonder what the penalty would be for a boat which did go through the start finish line on a downwind leg, when the S.I's state that this is not permitted. I guess the PRO could disqualify them for not sailing the proper course, another competitor could protest them, or could they return back over the line and go around the ends to exonerate themselves? It's not quite so clear cut as the Black Flag Rule.