In Japan, there are varying levels of respect when speaking with people. Her name is also NOT Hayase Nagatoro, it is Nagatoro Hayase. Names are in multiple parts of that respect hierarchy. Even among friends its typical you call people by their family name, unless they are really close friends, and family/loved ones are the ones who mainly only use your first name. It is all about respect. You cannot use customs and culture outside of Japan to negate anything that has happened in this manga, because it is based in and being written in Japan.