As an architect who has witnessed what blizzards can do to roofs around Denver, adding more weight, over what they were originally designed to support, is something I strongly advise my clients avoid doing.  Most things in design are a trade-off, and most commercial roofs are designed to minimum code standards.  Given that green roofs will tend to hold water, even in winter, I can see a big unintended consequence, if this passes, will be an increase in roof collapses.  Whatever energy savings that may accrue will be more than offset in rebuilding costs and lost business revenues.