IP addresses used in this case study are all in Private ranges (LAN,WAN, lo0). It is used for demo purpose only and I believe they cannot use Public range as that ip range may be registered by someone in the internet (legal issue).
As you thought, if you want to build a gre tunnel over internet the tunnel end point IP address should be Public range so that it can be routed by the intermediate devices.
Thanks, Nellikka JNCIE x3 (SEC #321; SP #2839; ENT #790) Please Mark My Solution Accepted if it Helped, Kudos are Appreciated too!!!
I really thought I was missing something. Indeed Nellikka, it could have something to do with privacy (somebody owns such an public ip-address).
Nevertheless, careless of Juniper not to mention that the ip-address should be public. Especially because, in het cloud, the word "Internet" is stated. Because tunneling can be used for traffic with non routable ip addresses I thought I was misunderstanding something, but now it's clear.