It's a hard thing. Like, from pure moral principles? No. I don't think priests should get any exceptions. But simply from balancing the positive and negative consequences of forcing them to testify? It's not worth doing. There's a lot of religious practices that wouldn't exist in my ideal world.
For example, in an ideal world, I wouldn't let parents force their children to attend church. Ideally religion would be something you actively have to seek out, rather than something forcibly imposed on you by your parents.
But I shudder to think at what horrors would be unleashed if you actually tried to ban parents from taking their kids to church. From their perspective, without baptism, their kids would be one fatal accident away from an eternity in Hell. That kind of pressure will motivate people to do unspeakable things. It is terrifying to think of what someone who truly believes, "you are trying to damn my child to Hell," is capable of.
You just have to do a cost/benefit on it. It is practical to ban certain religious practices. If someone wanted to build a pyramid and start sacrificing people to the Sun god, even willing people, we wouldn't allow that. But we wouldn't allow that because:
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The consequences of allowing it are extreme. (People are being murdered.)
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The consequences of banning it are minor. (Not many people actually want to sacrifice people to the Sun god. There's few enough of them that they do not present a threat to the rest of society. We can easily arrest the handful of people that engage in human sacrifice without tearing society apart.)