Pegging it to a Medicare rate isn't the lazy part. Allowing that rate to be less than Medicare is the lazy part. I really feel like you're not understanding how this works like you think you do. It is certainly a common way to write the contract, and the negotiation needed to agree on what amount to settle on is and can be a significant effort and can be heavily contested/negotiated. So it's not the contract design that's lazy. Accepting a rate below Medicare because you make enough on Medicare is lazy and puts business at risk if Medicare rates were to shift downward.
I'd not let my Managed Care team get away with that negotiation. If the other side wants to use Medicare as a baseline, then fine, reimburse at Medicare rates is the absolute lowest I'd ever go (Medicare +0%). Much more commonly, if we're using Medicare as a baseline it's going to be Medicare +20-50% or more above. Depends on the exact service and economics involved, very case by case. Going below 0% is a no-go for me and I can't think of a time I ever would have accepted it, so if you're job is to negotiate, then this is lazy. See what I mean by "lazy"?