Why are people allowed to proselytize with microphones at Seoul Station?

It is the gateway to Seoul, but every time I go to the station, I feel exhausted by the sound of people proselytizing..
I can't be the only one feeling this way, so why hasn't there been any regulation for years? If the law doesn't allow it, shouldn't they establish a legal basis for regulation?
With so many foreigners visiting, they sing and preach things like "O-something," and I can't understand a single word; it's just a buzzing noise. I find it incomprehensible every time I go there.
(Please do not misunderstand; I am not disparaging any specific religion.)