El Shaarawy declared his commitment in crunch talks with vice-president Adriano Galliani last week.
Berlusconi said: "With El Shaarawy we were favourably impressed by the fact that he showed a great attachment to Milan and he said he did not want to play for any other team in the world that was not Milan.
"When you see a team for the first time when you are young you have affection for them and Balotelli has become one of the heroes on the pitch.
"It is very difficult to change the team you support and I was born with this passion my father gave me.
"It would be sacrilege to cheer for another team, Mario was born a Milan fan and we are pleased about that."