Why is it so big deal if a non Muslim person uses ¨Selam aleykum¨

[Suzi87]

Hello, how are you? In this multilingual video

https://www.youtube.com/watch?v=CGVzORPuUwo

I spoke in Turkish with a man from Argentina... I just helped him learn this language for a bit...for him it is difficult to pronounce letter "H" because it is silent in Spanish. That is why he can not say "Merhaba". Then I said him what is familiar to all of us...in Serbia, in Argentina, ... All Muslims use it, so I thought it would be very easy for him to pronounce it. Plus, I wanted him to impress Turkish speakers with that... and when two Turks saw it they reproached me and said I could not use it if I'm not a Muslim. Then, why don't you in your textbooks don't put this sentence "Selam aleykum is forbidden for non Muslim people"

I told my friend about this "huge mistake" I made.

My mood is down because of this.