"Why did God reward Leah for allowing Jacob to sleep with her servant?"

Genesis 30:9-10 When Leah saw that she had ceased bearing children, she took her servant Zilpah and gave her to Jacob as a wife. Then Leah's servant Zilpah bore Jacob a son.

Genesis 30:18 Leah said, “God has given me my wages because I gave my servant to my husband."

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After Leah is no longer able to conceive, she offers her servant to Jacob as a wife so that she may have more children through her. Leah is then rewarded for her actions by being allowed to conceive children again. Why would God reward what is effectively polygamy?  

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Referenced Verses:
Genesis 30:9, Genesis 30:10, Genesis 30:18