okay, this is very good question!
well, itz not limited only in china who simply would call stranger or first meet as friend!
i'm surely tell you, this happen in most of asian custom.
this kinda expression, is our asian custom and lives' philosophy.
1) we are friendly from the inner for stranger.
2) we're simply making friends with others.
3) the word "friend" means, ppl who we just gota know 1 minutes we're start to used "friend" to call them instead of their name!
yeah, and so on.
when i talk to stranger esp westerners. they are frightened lotz becuz i call them "my friend" .
and at the same time, when they replied, "i'm not your friend" this word also surprised us(asian) too!
and we start to think this person is not friendly at all!