Rotors keep warping

[Flop]

Hi guys.

I have a '98 E320 wagon that I bought a year ago. I starting feeling a vibration and took it to an independent Mercedes shop. They determined that my rear brake rotors were warped. I checked the docs the previous owner gave me and he had had all the brakes completely redone 18 months before this. Seemed too early for warped rotors, but I went ahead and had them replace them. Two months and 200 miles later, they're warped again. The shop is replacing them for free, but says they won't do it again. They say it's my fault because I don't drive the car often enough. Is this possible? I have to park outside (in Boston) and only drive one or two times a month. This sounds ridiculous. I've never had this problem with any other car.