"Why can't I choose for myself?" they would ask. "Aren't we free agents?"
His answer was to compare them to "free agents" in the world of professional sports. When a star player's original contract expires, his services are "up for grabs." But once he signs with a new team, he no longer has the option of showing up in a different uniform, playing for another team.
"You were a free agent, but now you've chosen to sign with BYU-Idaho," he'd say (though I'm paraphrasing). "As long as you're here, you live by the team rules."
This can also apply to our lives in the church. When we're baptized, we've signed with a team. While we're on that team, we're expected to live by certain rules. We don't have to keep deciding every week which church we're going to go to, which callings we're going to fulfill, which commandments we're going to obey.