That sounds backwards. If competition were actually cutthroat, no ISP could easily demand payment from Twitch, because then Twitch would just say "Due to the unreasonable demand, Twitch streams will be throttled for SK Telecom users to 360p," and the other ISPs would advertise like hell: "Come to us and enjoy Twitch in its full HD glory!"
The fact that it doesn't happen is a strong indication that the competition is lukewarm, especially considering that Korean antitrust laws are pretty much toothless and companies collude all the time.
That's why SK Broadband (not to be confused with SK Telecom, a much more popular ISP that shares little infrastructure with SK Broadband) goes to the government and whines about evil foreign companies clogging up their pipes. They can't make the first move in the market, unless everyone else is required to do the same.
Korean telcos do collude all the time, but even here, a surcharge for consumers who were previously promised unlimited data is not something they can get away with.
The fact that it doesn't happen is a strong indication that the competition is lukewarm, especially considering that Korean antitrust laws are pretty much toothless and companies collude all the time.