Why do you think that would be the case? Freight dogs not being able to make pax announcements? ;-)
Those company who have the freighters on the same AOC operate both, pax and freighters if the have a freight and a pax version of the same a/c.
Those who have a separate cargo company, like Lufthansa which in addition doesn't have a pax version of the 777 operate only freight.
But they can switch back to the main line when they to their command or swap type.