In each of the seventeen states and DC, National Popular Vote Interstate Compact has been passed by the state legislature, and signed by the executive branch, except for Hawaii (whose governor tried to veto it but it was overriden).

The bills followed all the proper steps to be signed into respective state's laws. It's not some kind of "agreement." It's the LAW. If you don't like it, feel free to go campaign in those states to change the LAW.