University Grads, Full Staff; Poly Grads Contract Staff. Why?

Why do most employers in Nigeria employ university graduates as full staff while polytechnic graduates are employed as contract staff and are made to do the same work, sometimes more work are given to them to do than the university graduates?? If the polytechnic graduates were not good they should not be employed in the first place rather than employing them, giving them more work to do and paying them less than their university counterparts. What is your view about this situation??