Workers get tired of bailing out and subsidizing entitled co workers. If you have to work every Saturday morning because Jehovah’s Witness Johnny has field service you just might look for a different job. This terrifies your employer as he doesn’t want to cover every weekend either.
My brother is an elder. When he went out to look for employment, he would always tell the hiring manager that he was a Jehovah's Witness. That meant that he couldn't work late on Monday's and Wednesday's. He couldn't work on Sunday's because he needed to attend meetings. He also needed a certain week off for summer conventions. Most companies wouldn't hire him with all those stipulations. The company that did hire him gave in to all the rules, but within time they made him work on a few of his meeting nights. He quit after they decided they wouldn't give in to all his rules on when he could or couldn't work.