Working 40h weeks for an extra $500 isn't a good deal. As long as I can sustain life without working, not working at all will always beat getting some scraps. If there aren't repercussions for not working, than why bother?
The idea is that you'd be able to sustain if you lived _very_ frugally. Generally, people don't enjoy that. They like to eat the nice food, go to places, buy things, all of which wouldn't be covered by the UBI.
I lived my entire live frugally. I enjoy that. Working 40h weeks on the other hand isn't all that fun, even though now I have thousands of dollars to blow every month. If I had the choice I'd live frugally and not work.
I'm eating the same thing everyday, like to stay at home and hate to buy things. I'm working because I know it's necessary.