The customer cannot receive service, so they're not getting anything for free. I don't know if anyone is arguing the contract is void (I don't believe they are); rather, some have suggested that this is a poor business practice. This situation is likely one that is not encountered terribly often. And this suggestion is not unprecedented -- at one point, cell phone companies were known to allow customers out of contracts if the customer moved to an area that was not covered (and there not only are we talking about a device that would still work in other locations, but about truly subsidized equipment that the customer owns and can resell).
Like I've already stated, the unilateral contract here is still valid. I just think enforcement of the ETF in this limited context is not cool. I feel the same way about Dish's policy of charging customers to return equipment (a fee I called and had waived the last time I ended service with Dish).