Is this legal? They are basically refusing me treatment unless I pay the higher "insurance" price.
It is legal, since the "list price" is likely much higher anyway and they're not obligated to provide any discounts. Many doctors offer "cash" discounts or special discounts for uninsured, but they're not required to.
But more importantly, the doctor's office most likely has a contract with that insurance provider (i.e.: they're "in network"), and if so - they're obligated to charge the insured by that provider the negotiated prices set in that contract.
Once you're identified as insured by that provider, you're covered by that contract and the doctor may not even have a say as to how much you're going to pay. They cannot "unsee" your insurance card, so they cannot offer anything to you that is not agreed upon with your insurer.