There is not Constitutional authority for a state to leave unilaterally either. You claim that since it is not prohibited then the 10th Amendment allows it. Well nothing prohibits the states from expelling another state against its will either. Then shouldn't the 10th Amendment allow for that? Why one and not the other?
The states do not derive their powers from the US constitution. It is very much the other way around. You seem to fail to grasp that. What this means is that the states do not require permission from the federal government to exercise a power not specifically stated in the constitution - such as the right to unilateral secession. The power to expel another state from the club by other states would have to be spelled out in the club bylaws. It isn't. Ergo, that individual state's right to free association prevails. They can remain in if they so choose. They can leave if they so choose.