If they really had no self-interest, as you seem to suggest, then the sign they left in the Nazca site would have just read "Time for change-The future is renewable" without mentioning "Greenpeace" in huge letters. You can't be so naive as to think that Greenpeace simply advocates issues of climate change, anti-oil, deforestation, etc. Their brand thrives on those issues and it's what defines them. Just look at their track record of getting people's attention with publicity stunts... How is that not self-promotion? In marketing terms they are brilliant.
The sad thing about this recent stunt is that Greenpeace is getting a lot of publicity out of it. Their so-called apology to the Peruvian people was a joke. Here's a direct quote from their official apology: "We fully understand that this looks bad... we came across as careless and crass." Looks bad? We "came across as"? They are apologizing that they stained their own image. They are sorry that the world now sees them negatively. In other words, they are expressing that they are sorry for themselves.... How far up one's own ass must one's head be for them to write something like that in an apology to the people of a nation? It should have read "We have no words to express the shame we feel for having acted the way we did." But that would be too humble.
If they are really sorry to Peru they would take part of their operating budget for the next few years to pay for all reparations and maintenance costs of the Nazca site.