[Sherman Logan]

I must admit skepticism that they have permanently scarred the site.

It’s been there 1500 years, and until very recently indeed anybody who showed up walked around without paying any attention at all where he stepped.

Although I do enjoy bad press for Greenpeace this all seems a bit hysterical.

[E. Pluribus Unum]

They entered a restricted area. What happens to you in the US when you enter an area designated as restricted by the feral government?

http://latino.foxnews.com/latino/lifestyle/2014/12/12/greenpeace-director-in-peru-to-apologize-and-help-investigate-nazca-lines-stunt/

The activists entered a "strictly prohibited" area where they laid big yellow cloth letters reading: "Time for Change; The Future is Renewable." They said after initial criticism that they were "absolutely careful" not to disturb anything.

Castillo said no one, not even presidents and Cabinet ministers, is allowed without authorization where the activists trod, and those who do have permission must wear special shoes.

[Zathras]

You are very correct that its unlikely the damaged anything.
However World Heritage site are probably some of the closely managed places on earth.
I’ve been to two, Petra in Jordan and SS Yongala off Australia.

Everything is considered “No Touch or removal”
To violate this can easily get you prison time.
Greenpeace is in a great deal of trouble.