What can be their reasons?
Whatever that is/are (P.S - nicely explained in the other answer by mhoran_psprep), you have a very solid reason to decline this and continue with the job search - this is not normal, if not a scam altogether.
They can have a thousand reasons / justifications, but you don't have to listen / comply to any of them. An employment is a two way street, if one party is not going to be happy by the agreement, the relationship is not going to last long anyways.