So, someone being active on a stationary bicycle while reading is wasting someone else's time somehow?
I've never found an enjoyable workout activity. The closest I've come to it being bearable is walking as transportation - and sometimes that leads me to staying home rather than do social things. It is horrible without headphones and horrible in the rain or snow.
All of these "workouts" and "exercise" have mind-numbing boredom as a side dish.
By trying some other activity instead of wasting their time and other people's.