In the last episode of ,
Brian said:

But Harrell said airlines including United, Alaska and Lufthansa still
let him accrue miles when he buys a second seat for his cello.
Limiting the amount of miles passengers can collect translates into
profits for carriers, he added.

Limiting the amount a miles passengers can collect translates into
passengers limiting their travel on that airline, which costs an airline
more than giving out miles for purchased tickets.

I'm having trouble understanding what the airline's problem is here.
He's paying for the seat, why can't he have the miles? If he wanted to
consolidate the miles into one account I could understand the friction,
but if they're maintained and managed separately, for tickets purchased
and flights flown, what's the issue?

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The nice thing about standards, there is enough for everyone to have their own.