By Nandita Bose and Kanishka Singh REHOBOTH BEACH, Del. (Reuters) -U.S. President Joe Biden said he was “proud” of Apple Inc workers in Maryland who voted on Saturday to join a union, becoming the first retail employees of the tech giant to unionize in the United States. “I am proud of them,” Biden told reporters on Monday. “Workers have a right to determine under what condition they are going to work or not work.” More than 100 workers in Towson near Baltimore “overwhelmingly voted to join the International Association of Machinists and Aerospace Workers,” the union said on its website on Sat…