Once those domestic workers become HK permanent resident, they have to pay tax according to the HK tax system. They are eligible to the education, health & housing system as they are HK taxpayers. They contribute their time & effort in HK, they pay tax in HK, it is fair to be covered under the same welfare system. In addition, the domestic workers might not qualify to be HK permanent resident after the overall evaluation process.
Let's compare the contribution level between those domestic workers and those 內地孕婦. No doubt the domestic workers play an important role in HK. How's about the 內地孕婦? Any contribution? On the other hand, they would like to enjoy the benefit purposely.
If you say it is unfair to them, I would say it is unfair to all HK people.