They are (probably), just not unilaterally. If the federal government (Congress) passed a bill allowing it, who would be there to stop it? This is how it is in most countries.

Ultimately, “legal” isn’t a very good claim to make on these questions… a secession that is successful will obviously be considered legal by the seceded country.

You may find this interesting: astralcodexten.com/…/who-gets-self-determination