Everyone has a choice whether to shun someone or not. The WTS only has the power over you that you give it. The WTS can't shun you - how many people have daily dealings with the WTS really? Do you go for coffee with the governing body? Of course not.
The people who decide to shun you based on what the WTS beliefs are - they are the ones that shun you, and that is their decision. Every decision people make is based on some external influences. You think you can legislate life to that degree?
There is no "simple" law to just make it illegal. Seriously, try coming up with the law and it'll be easy to demonstrate ways it can be bypassed or misused.