> If "thinning down the population" was the intent, shouldn't the US reverse their anti-abortion bullshit laws?

There can be more than one motivation, of course.

Also, it's very possible that outlawing abortion can result in a decreased birth rate. E.g., if a woman is scared that she'll have a problematic pregnancy, she may decide to just not have a child instead of risking needing and not being able to get an emergency abortion.

The GOP stance on abortion is about capturing the religious voters, which is far more important to them than anything else.

Those are ideological and motivated more by wish to punish women who "did something immoral" then by wish to have growing kids.