Originally Posted by
pinniped
Actually, in the U.S. it generally isn't: you're forced to buy your miles along with your revenue ticket whether you want them or not, with very few exceptions.
You do not provide a number, you do not get miles. What's the problem?
If they want to enroll you every time to give you miles, just forget for the the number and you are all set.
Noone is forced to get or use miles - you can simply fly and not care about the FFPs...