In this case expiring is not the same as airline miles. The product doesn't run itself. If you only want to use your credits on the odd numbers months and you expect it to be there and ready, well it still has to be there and maintained on the even numbered months.
I guess you can bake that into the credits price. But if you say, purchased 100 credits today and don't use it for a year...well, who is paying for that year? Adding a small monthly fee, oddly enough, keeps the customer engaged.
Airline miles are a perk.
