The mortal girl has nothing to recommend her but a pretty face, stubbornness, and the fact that she's carrying Tam Lin's child.
Pretty faces the Queen's court has in plenty, and stubbornness the Queen has unmatched, but blood calls to blood, and that child is as fae as her father. The mortal girl can draw on the babe's power—and when the Queen had shaped Tam Lin to be a fae knight however mortal born, she had given him quite a bit of power.
The Queen surrenders Tam Lin without more than a token fight. It is, after all, the night of the teind, and she would lose Tam Lin either way. Better to lose him to a mortal girl than to hell; the Queen has a chance of stealing him back from the mortal girl.
The girl weeps with joy and imagined triumph, and the Queen turns her thoughts to the knights remaining to her, and which to give as tithe.