I rechecked the scene just now, and my problem with the American Express machine wasn't so much that the company still existed as that it was actually a traveler's cheque dispenser, rather than an ATM (cashpoint) as I thought. Even at the time, it seemed unlikely that traveler's cheques would still be in use in the 23rd century with its implicitly credit-based economy, and indeed, their use has been in decline in the decades since the book came out, due to the wider use of credit and debit cards and ATMs/cashpoints. So it felt at the time like it would make the book feel dated someday, and it already has. Also, having it be an American company, on top of the Centaurus culture as a whole being so indistinguishable from 20th-century America, just made it feel too quotidian and unimaginative. Plus, Earth's supposed to be one global nation in Trek's era. Okay, the USA still exists as a geographic entity, but it's part of a larger whole. I guess maybe it's analogous with a company name like Texas Instruments or Ohio Art (the original manufacturers of the Etch A Sketch), but it still felt incongruous.