I think you have answered the question. These are loyalty programs, they are not intended to reward past behavior, but rather to influence future purchases. If you are promiscuous and only patronize a particular vendor infrequently the sponsor clearly sees that the program has not had the intended effect.

Don't misunderstand, I'm not saying there is anything wrong with using the programs in a way that doesn't infuence your choices. Nonetheless, I understand why the programs choose to purge individuals who are not engaged with their brand because that's the primary justification for the programs.