TITANsmurf brings up a good point. If it was the regular warranty itself, you should be able to do you own maintenance as long as you show that you did indeed perform the maintenance (with receipts,etc).
However, since im pretty sure Nissan itself doesnt give a lifetime power-train warranty, the dealer probably has a stipulation itself (separate from Nissan), stating that in order to receive the lifetime warranty (which im guessing it provided by the dealer and not Nissan), you need to have all the maintenance done at the dealer itself. Im taking a guess that this is what they are trying to do to you.
Again, my GUESS is that the dealer provided you with a lifetime powertrain warranty through the dealer in order to keep your maintenance business with them. The probably figured they would make their money back by forcing you to have work done only with them in order to keep the warranty (the separate lifetime warranty, not the Nissan warranty) intact. Like i said before though, it is hard to state that this is true without more information about the lifetime powertrain warranty they provided to you.
Do you have any paperwork stating the details of the lifetime warranty?
