Originally Posted by flyerCO

Never said that was the sole reason. However it is a reason. This gets argued periodically, but generally each time an accountant or lawyer weighs in, the view is that it would be treated as income. Income that would be easy to track and wouldn't be so small as to be written off as insignificant. Miles are basically a currency.

Think of it like if you got Bitcoin instead of miles. (Assumimg in this case you owned the miles, like you would the Bitcoin) You'd be expected to pay taxes on the income. $5 in value here or there they really could careless about. But we're talking hundreds/thousands of dollars in value for those earning lots of miles a year. Also easy to track who has earned what. Basically if we owned the miles, it would be a bank account, but in miles instead of dollars.