Is it ok to sell weapons of war to a country engaging in ethnic cleansing, and if not - why not?

After all, the country is allowed to decide it's own rules and laws. If it decides ethnic cleansing is allowable, we should follow that right?

Of course not! Just because a country decides some action is legal doesn't make it moral or ethical - and knowingly aiding an unethical act is itself unethical! We do and should absolutely shun and punish countries engaging in things like ethnic cleansing - even if they're assisting a country that has deemed it legal.

Why in the world would religious prosecution be some kind of special case?