> Why would an airline buy an Airbus

Because they're the best planes available?

> or a Boeing branded Airbus with Boeing markup?

Because the Boeing markup is less than the US import tariff?

People do strange things, like buying John-Deere-badged excavators manufactured by Hitachi, or JD-badged dozers manufactured by Liebherr.

Or JD-badged lawnmowers built by MTD.

They’re already buying an Airbus branded Bombardier with an Airbus markup so why not?

Because they have no choice. There are currently 2 aircraft companies in the world that make commercial jets: Boeing and Airbus (we'll ignore Comac for now). If Airbus somehow bought up all of Boeing's manufacturing facilities and capability, leaving Boeing as just a re-badger, then airlines would have no choice but to buy an Airbus that either has an Airbus badge or a Boeing badge.

In a word ---- marketing.

For the same reason people currently buy "American" autos (and planes) built from parts sourced around the world.

In American management culture, marketing is the knee jerk solution to any problem and the holy grail from which all revenue is gifted. They are true believers in the "magic" of being able to "game" people into making decisions based on something other than logic.

This same culture explains how/why the internet we have today is built around privacy invasion and (wait for it) ... marketing.