U.S. airlines are wading deeper into the charter business to Cuba as travel restrictions have loosened, hoping one day to haul American tourists to the island’s unspoiled beaches.By year-end, four of the largest U.S. airlines will operate about 25 weekly flights to Cuba for charter companies. AMR Corp.’s American Airlines has been flying to Cuba for two decades, and JetBlue Airways Corp. arrived last month. The world’s two biggest carriers by traffic, United Continental Holdings Inc. and Delta Air Lines Inc., are resuming weekly service to Cuba after a seven-year hiatus.Charter companies estimate that they will fly 400,000 …