Which is precisely the law Hagar is assumed to have broken in Sarah's eyes...
146. If a man takes a wife and she gives this man a maid-servant as a childbearer and the maid-servant does bear him children, then this maid assumes equality with the wife; because she has borne him children, her master shall not sell her for money, but he may keep her as a slave, reckoning her among the maid-servants.
141. If a man's wife, who lives in his house, wishes to leave to engage in business, plunging them into debt, trying to ruin her house, and neglecting her husband, and is judicially convicted, if her husband wants a divorce, she may go on her way, and he gives her nothing as a gift of release. If her husband does not wish to release her, and instead chooses to take another wife, the first wife shall remain as servant in her husband's house
It's not talking about equality of provision, but status.
One problem with inequality of status is that it often leads to inequality with provision.