In most cases, I don't think FF changes are unethical. Sometimes I question whether they make good business sense, but that's a different story. I tend to agree with Efrem's gift-certificate analogy, even though I don't like it as much as I like to think of miles as cash.

I don't think that the airlines are unethical: I just think they are poorly managed and tend to do stupid things without thinking about how they will affect the customer.