The larger concern is they use SPLC's money to pay for venues for those sermons, or direct SPLC's resources away from SPLC's goals because those goals are counter to the individual's politics.
Companies ought to have the right to fire people acting counter to their goals.
It's not "SPLC's money" once they've paid it to someone else for labor, any more than it was Planned Parenthood's money when a Catholic donates it to a church that gives sermons against abortion.
Companies ought to have the right to fire people acting counter to their goals.