What's voluntary? States don't join the Union, they are admitted. And, constitutionally, they can be admitted without any input from the people in the territory at all. Likewise, the people of the territory can petition till the cows come home and they will become a state only when Congress allows them to.
DUH...It's being forced to leave against your will!
DUH...and where does the Constitution say that the states cannot do that?
It doesn't. Nor does it say they can.
That's because the States are FREE TO CHOOSE if they stay OR leave the compact. The federal government has NO say-so either way.
That's what makes it VOLUNTARY!