Please explain how preventing the JWs from ENFORCING the shunning practice (that is, excommunicate people who do not practice it) would undermine their freedom of association.
People have the right to form groups, with the rules they see fit. Others can join and agree, or leave. If the group is forced to behave, not in the way the members agree upon, but rather the way YOU think they should behave, and they have no recourse ... because accoring to you they can’t enforce the rules, then there are a myriad of rights being trampled on, one of which is freedom of association. (They can’t kick out people associating with other people they see as “bad”).
Again, I am not saying they are moral for doing this. They could be ass holes. But you can’t legislate away ass holes.
Then compare this to the freedom of association of the people being shunned by their entire social circle all at once.
Yes, the person that left the group has great freedom. He/she left!! Even if they were kicked out, they had the freedom to break the rules and the group exercised its freedom not to associate.