Most of them object based on religious grounds. They consider it religious discrimination if an employer is able to fire someone in such a situation.
Oh, I'm aware of their faulty reasoning. In my opinion, anyone whose religious duty interferes with his job duties ought to choose which duties are more important and live with his choice. Christians and Jews should be especially mindful of the need to make "hard" choices like that, if they read their bibles. Given that I should be willing to die rather than compromise, it doesn't seem like it's asking too much of me to look for another job.