A large city in your average state can get away with a lot more corruption and mismanagement than DC could. They'd have state government, certain national programs, and national representatives to fall back on. DC gets none of that.
I agree that companies price these cards expecting to be able to reclaim a certain percentage of the value of these cards. However, they are selling the entire value to the consumer.
So if (and it's a big 'if') the unused minutes are unclaimed property, then gov't escheatment isn't a tax, it's reclaiming lost property on behalf of the consumer. The consumer paid for the entire value of the card, not the value of the card minus a bonus for the issuing company.
Anything left over on a card should be treated as pure bonus to the companies, not a standard revenue source that needs to be recouped. If they do raise rates to recoup lost revenues, then it is the companies performing a cash-grab, not the gov't. This is why companies like gift cards and pre-paid minutes cards... they've been able to extract out the unused value from the cards for years, and it's all "extra" money that they didn't have to earn.
Clearly there is enough grey space to go to court about it.
It's true that they could declare a law against expiry, but that would seem to be a completely separate case.