Why is there a helicopter visibility reduction restriction for this approach?
Helicopters may operate under special VFR when in flight visibility is less than 1 SM.  Therefore, the approach can be flown at lower visibility than that published on the approach plate.  However, the note must be published to further restrict these types of operations:  "The procedure is annotated with 'Visibility Reduction by Helicopters NA.' This annotation means that there are penetrations of the final approach obstacle identification surface (OIS) and that the 14 CFR Section 97.3 visibility reduction rule does not apply and you must take precaution to avoid any obstacles in the visual segment."1