“In addition to being transparently political, these provisions have negative practical implications. For example, the proposed rule setting an arbitrary 10 percent price-increase threshold could cause insurers to target rate increases to just below the limit. Indeed, it is a well-documented effect of price controls that sellers respond to the imposition of price ‘ceilings’ by turning them into price ‘floors.’ The less competitive the market on which price controls are imposed, the sooner that phenomenon occurs.”