Well here in the UK you can have the same company employing people to do the same job and paying different rates. In this case they would not want employes to share what they earn for obvious reasons.
But I suppose mainly the reasons would be not wanting others to know if you earn less than them, a sort of pride thing i guess.

Thanks for the reply midlandi.

for my opinion so that they will not compare and have some doubts why they have different salary