If UA were smart, it would do everything it can to make sure this works. It's basically "free" money for UA courtesy of Amex.
What you're forgetting is that to post as an incidental charge, it won't post as a flight. Some people are more interested in it posting as an incidental charge (for the reimbursement) but other people are more interested in it earning a multiplier on cards that earn multipliers on flights.
So there's no one way that UA can set this this up one way to please everyone. And why would they "be smart" to prioritize people who have UA selected as an incidental charge for an Amex card, when there
may be way more people who are earning some sort of multiplier for UA flights (which is possible on various cards for various banks, including some Amex cards)?