Kat, the absentee ballot requirements vary from state to state. Some states, like NH, require those who apply for absentee status to swear that they will be away on election day, or have a disability that prevents attendance at the polls. Others allow any registered voter to mail it in. And still other states allow voting to take place in advance of the mandated election day.
It's one of the quirks of our system that all elections, even those for federal office, are supervised and conducted by the several states, and not the federal government.