Actually, this is a good decision.
There are a several states that have Republican governors and Republican legislatures that could vote for a Democrat president in the General Election.
This ruling means that Electors from those states would be free to follow the wishes of the voters who elected their state leadership and vote for President Trump in 2020.
Each party has its own set of electors. When you vote for Trump, you are actually voting for his slate of electors who are chosen by the Rep Party. Some states require the electors to vote based on the state popular vote. Other states don’t have such requirements. This federal decision overrides the State’s ability to require how an elector votes.
And vice-versa.