Well, maybe I solved the mystery
I was looking aboute maritime services and their network. If you check
the list of MCC (Mobile Country Code) you can note that only few maritime networks are using a Satellite code (MCC 901):
Other networks as Isiminn or Tim on board also are using the usual national codes and so United Mobile (and others) can't know the different way of system using the usual tariffation plan. So if you roam on Tim or Isiminn maybe you'll have your usual roaming charges, but when you roam on the code 901 (it's a satellite code, pay attention!) you'll have the extrasurchage. What do you think? Am I wronging?
