Laban wanted Jacob to have an exclusive relationship of legal marriage with his daughters. But the OP is mistaken if it presumes that Abraham had many  many wives simultaneously. He married Keturah only after Sarah had died. Reading the last verse of Gen. 24 together with the verse of Gen. 25, we find: 

> [Isaac] took Rebekah as his wife. Isaac loved her and found solace
> after the death of his mother. Abraham took another wife, whose name
> was Keturah.

Hagar is said to have acted as a "wife" but her status was that of a concubine for she remained Sarah's slave. (Genesis 25:12) Her position was the same as Bilhah and Zilpah, the slaves of Rachel and Leah, who were Jacob's concubines even though they were also called "wives" (Genesis 30:9, Genesis 30:4). 

Thus Laban's action in insisting that Jacob "marry" only his daughters was not much different from that action of Abraham - who did not actually marry Hagar and married Ketubah only after Sarah died. He did trick Jacob into marrying both of this daughters, but Abraham did not have more than one legal wife at the same time.