The fun of law, engineering, and arts is a matter of taste. I am not sure if that applies to flipping burgers and delivering pizza. I know a lot of people developing software for free just for the fun of it. I doubt there would be that many people flipping burgers for free (or a very low salary) if there would be an unconditional basic income.
Developing software is fun. Developing the software that they want you to develop usually isn't. You don't get paid to develop software; you get paid to develop the software that they want you do develop.
