It may be advantageous for a company that manufactures its own aircraft to also be an airline. For example, the airline business would likely have reduced maintenance and parts costs relative to other airlines, and there would be a tighter feedback loop between manufacturing and end consumers.
I have read that Boeing, who used to own United Aircraft and Transport (UAT), was forced to dissolve UAT after investigation into the scandalous "spoils conferences" (source):
In a move clearly aimed at Boeing, new Postmaster General James Farley and, later, Congress banned the award of the new contracts to any companies that also built aircraft.
I managed to find what I believe is the Air Mail Act of 1934 that supposedly bans these vertically integrated companies here, but I don't see where, specifically, the act bans these companies. It's unclear to me whether this was a one-off ban of combined aircraft manufacturers and air mail companies, or whether this ban continues through today with modern airlines. I am specifically interested in the US here, but would also be curious about other countries (e.g. could Airbus do this?)
Is it still illegal for all manufacturers to start their own airlines, or does this law no longer apply today?