“There is nothing in the Constitution that forbids the states from directing the electors how to vote.”
The US Constitution does not work that way. It works by defining what the Federal and State governments are allowed to do, not by defining what they are not allowed to do. It can’t be otherwise, because it would be impossible to list every possible disallowed action, creating a loophole of infinite size for all sorts of tyrannical actions.
Hamilton makes that explicit in Federalist #84.
“The US Constitution does not work that way. It works by defining what the Federal and State governments are allowed to do”
That is true for the Federal government (10th Amendment) but not for the states.