On being a public nuisance...

So, last night, my friends and I had had a few drinks before leaving the apartment to head to Hongdae. We were on the metro and maybe a song or two arose among us. OK, OK, we were singing on the train. It might have been a little obnoxious, but honestly, if I saw a group of girls singing on the train at home I'd just laugh. Would anyone walk up to them and tell them to shut up? I can't see it happening... This was our experience:

Us: lalala (singing some song or another)
Drunk Ajossi getting of the train: ㅓ이랴ㅓㅣ패ㅑ더리ㅏ내ㅑㅓㅍ디ㅏㅜㅍ니ㅓㄹ그ㅜ피ㅓ대ㅓ대ㅓㄴ나ㅜㅐ퍄ㅜ디ㅏㅓ래ㅑ덜ㄴ러너랜ㄹ피ㅐㅑ더래 ( angry Korean speaking)
My friends: (stop singing) Jo-Anna, what did he say?
Me: I duno... probably shut the hell up...
Drunk Ajossi getting of the train: ㅓ이래푸패ㅑㅓㅜㄴ이ㅏㅓ내ㅑㄴ파ㅜㄴ래ㅑㅓ대ㅓㄴ (more angry Korean speaking)
Us: Laugh and continue to sing.
(Drunk ajosshi gets of the train)

Second Drunk Ajossi: (walks over to us and stands right in front of us) ㅏㅣㅓ리ㅏㅓㅣㅏㅓ (some more angry speaking)
My friend and I: Sorry, I don't understand....
Second Drunk Ajossi: (thinking very hard in his drunkeness) Many.... talking.... no... like.
My friend: 궨잔아요 (it's ok)
Second Drunk Ajossi: 안궨잔아요!!! (it's not alright!) ㅓ퍼ㅏㅣㅓ채ㅑㅏ 알아요??? (something something you know??? something something you know??)
Me: It's better to pretend like we can't understand... Just ignore him...
Second Drunk Ajossi: uh..... Welcome to Korea!
Us: (to ourselves) are you kidding me?
Second Drunk Ajossi: Where are from??
My friend: 미국 (USA)
Second Drunk Ajossi: ㅣㅏㅓ나ㅑㅇ (clearly not registering that we're speaking his language)
My friend: 미국 (USA)
Me: Give up.

Around that point we got so creeped out by him we moved to another car... and stopped singing...

I would like to know where that "Welcome to Korea" thing comes from anyway... is that what the government has trained them to say to forigners??? They all say it! I've never gotten that in any other country in the world... especially not from complete strangers on the subway... Even the sober ones...