The Supreme Court's job is to intrepret law, not to make law.
The issue is kicked back to the states because of the 10th Amendment which states:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
Garbage. This wouldn't be to make law. Right to Life is laid out both in the Declaration of Independence an the Constitution ... and is foundational to all the rights laid out in the Constitution. By claiming that this particular decisions rests with the states, it's stating that pre-born children do not qualify as life, but are, rather, something else.
If a state were to pass a law declaring that murder is permissible, the SCOTUS could (would and should) strike that down as Unconstitutional. That would not be to make a law, but to lay down the parameters under which laws can be passed. Any law that would legalize abortion would under a properly-formulated ruling would be contrary to the rights of the unborn and would therefore be unconstitutional.
This is stupidity to claim that such a ruling would tantamount to "legislating".