You buy something for $100.
A 2% CB card gives you $2 cash back, non-taxable purchase rebate.
A 2mpd card gives you 2 miles as a purchase rebate. You sell to a broker for $3. Why is that $3 not a purchase rebate from your $100 purchase, just like the cash back is? I'd argue it is.
I hear your argument, but I would counter, that once you got the rebate
which in this case in not in the form of cash/currency, but rather something else (a commodity?), and now you sell it for more than the bank would credit you for it (IINM Chase allows you to use UR at a value of 1 cent towards your balance) which could be considered true FMV, as T&C don't allow selling, the difference between what the bank would give you for it, and what you sold it for could be a capital gain.