I agree with you that abortion is a sin. However, imagine the following situation with me:
You are now the president China. A country with a population of over +1.3 billion people. Would you allow people to have their choice on how many children they can have? What will happen if people started having as many children as they want?! It would be a disaster; where would you feed those people from? How will you be able to provide schools, hospitals, libraries, etc. for your people? How will you be able to provide jobs for all those people?

etc.
Don’t you think that it is really hard not to put such policies?!