It's easy for an electronic voting machine to print out a vote receipt for a manual recount in case of machine failure or hackery. It's not like we don't know whether or not the machines can be tampered with and are hackable, it's a fact, they can be tampered with and they are quite clearly hackable. It was also thought that since the government is a public body, the voting system should be public as well, an open project that can be reviewed by any citizen who wishes to make sure their vote is dealt with securely by an electronic program.
However, the fact that the company repeatedly, refused to divulge details on the software (presumably because they wish to quell any distrust of the voting software), and during software demonstrations they didn't let anyone touch the machines or try various things out. In addition, leaked memos have shown that Diebold were using uncertified machines in production, that's machines running versions of software that haven't been approved for use (not that that certification must do any rigorous testing considering the shape the software is in) and who knows what flaws, back doors, or whatever have made their way in there.
The fact that receipts aren't mandatory and the fact that they're going ahead with these shitty machines with a shitty system to register your vote just shows how unseriously the american government is taking this voting procedure and almost guarantees problems. How many people are going to try to hack these machines on election day? It's going to be nuts.