It only costs money to provide "new" receivers to people in exchange for their obsolete receivers if Directv has to pay to manufacture actual new receivers as a result. Are they still manufacturing SD receivers? I'd personally be a bit surprised if they were.
I would think there would be enough D1x receivers coming back from people swapping them out when they upgrade to HD and/or Genie that they have enough on hand to do this. That may be why they aren't pushing everyone nationwide to upgrade all at once with a single national deadline, but doing it in phases over time. They know the rates at which the D1x receivers are coming back from these upgrades, so they can plan for that when doing the MPG upgrades.
Even assuming it costs Directv money (aside from the shipping and occasional lost customer who is upset they can't keep their 15 year old receiver) perhaps it will save them money in the long run not having to support the old guide. I don't know what that costs, but transponder bandwidth certainly isn't free. There's no reason for them to do it just to piss people off, there must be a business case for doing so. Either simple profit/loss or other improvements that don't necessarily show up on the balance sheet like no longer having to support receivers without a RID.
SL5, 2xSWM16, 16 H20-100, 1 H20-600, 7 H24-700/AM21, 2 H10-200