If it doesn't make sense to you, that the people doing the most productive (or reproductive) labor are also the most-likely to volunteer, then it's because you don't understand how transferable effectiveness and efficiency are, and how work tends to spread to fill the time alloted to it.
Someone with 5 things on his list for that day, will find it less difficult and less painful to add one more thing, then someone with nothing on his list. For the person with 0 things on his list, it's a 100% increase in effort. For the person with 5, it's a "mere" 17%.
The problem is that the person with 6 things will eventually get run into the ground, if he just keeps adding One More Thing.
And this is why UBI is so civiliation-destroying. It effectively means that you're discouraging people from moving from 0 things, to 1 thing. They won't be doing Great Works and Amazing Inventions with their newly-won time. Those will be done by the same people who always do them. The same people who do everything else.