If you bought the Tivos from a retailer then DTV should have sent you the cards at no cost to you. I've activated numerous DTivos and told DTV that they were open box items from BB or CC. They sent me a free card every time.
The CSR told me that the replacement refurb would come with a new access card, and it did.
I'm sure there must be some extenuating circumstances for your situation. DTV has never sent me a new access card with a refurb replacement unit. There's really no need for a new card since they can easily port the old card over to the new unit. I have never had a problem getting a card swapped from a previously subbed unit to a replacement unit.
The only time I've ever heard of DTV balking at activating a previously activated card is if it was previously on a different account, and even then they have been known to make exceptions. It really all depends on which CSR you talk to. I think some of them just don't understand the process involved with swapping cards between accounts, even though it can apparently be done without too much difficulty. You'd think they'd be more than happy to recycle access cards since it means that there's one less card in circulation that could get into the hands of potential DTV hackers. It is quite obvious that many CSRs just aren't all on the same page when it comes to certain things.