This is a fairly new practice--devised by insurance brokers, by the way. It may not seem at all fair; but it is legal to do.
While benefits are considered part of the compensation package when employers calculate the total cost of each employee, and while an employer is obligated to pay workers fairly for doing the same work, there is no specific legal requirement that the employer pay the employee anything extra for waiving insurance coverage, even though the employer required that the employee do it.
Now having said all of this, who knows what the future will hold? Laws may be passed to stop this practice at some point. But as of right now, there is no legal reason for an employer not to do this.