Massachusetts hospitals will get $275 million more a year in Medicare payments because of a provision inserted into the nation's new healthcare law that benefits the state, angering hospitals across the country that will get less money as a result. Medicare adopted the change last week, as required by an amendment to the health care law cosponsored by Senator John F. Kerry, a Massachusetts Democrat. The amendment essentially requires Medicare to reimburse all Massachusetts hospitals for employee wages at at least the same rate that it reimburses Nantucket Cottage Hospital, a windfall that Partners HealthCare set in motion in 2007, when the Nantucket hospital became its subsidiary. Wages on the island are hefty because of its isolated location and high cost of living.