Maybe if there is a real curse, Snape asked Voldemort about it and got a anweser along the lines it does not affect death eaters or here how to break it. Or maybe the curse is a myth, at least two people who appear to be affected work for Voldemort and thus should not be affected unless Voldemort being very stupid or thinks he going to get what he needs before the year is done and willing to take a cursed job because of it. the curse might be just a fluke the rumors of a curse made only the desperate take it and the desperate do not keep the job for long thus making the curse a self-fulfilling prophecy. Weirder things have happened in real life, look long enough for a curse or haunting and one is going to find one.

Maybe, if there is a real curse, Snape asked Voldemort about it and got an answer along the lines of, "It does not affect Death Eaters", or, "Here's how to break it". 

Or maybe the curse is a myth. At least two people who appear to be affected work for Voldemort and thus should not be affected, unless Voldemort being very stupid or thinks he is going to get what he needs before the year is done and is willing to take a cursed job because of it.

The curse might just be a fluke. The rumors of a curse made only the desperate take it and the desperate do not keep the job for long, thus making the curse a self-fulfilling prophecy. Weirder things have happened in real life. Look long enough for a curse or a haunting and one is going to find one.