It is not arbitrary, the very decision to limit high intensity workouts including the BPM restriction is coordinated between the government and gyms (the Korea Fitness Manager Association in this case) [1].

[1] https://www.nocutnews.co.kr/news/5586611 "거리를 두고 기본적인 방역수칙 외에도 특히 차단해야 되는 부분들에 대한 수칙을 만들기 위해서 애를 썼고, 관련 협회들에서도 그런 측면에서 어떤 것들을 좀 차단시키고 노력하겠다, 라고 제시한 수칙입니다." (Rough translation: In addition to the basic prevention measures like social distancing, we have also tried to make a list of especially required additional measures that the related associations have agreed upon with that context.)