[tene]

The usual justification for this is that it's simpler, and reduces additional incentives to cheat the system and additional overhead. We currently have a means already in place to claim a portion of an individual's income, through taxes. More-traditional means-tested welfare systems have additional overhead of a second system trying to investigate your income to determine how the government chooses to interfere in your finances. The proposed intuition is that you get a simpler system with less overhead by just giving out a flat UBI payment, and also claiming some amount of that back via taxes that people already file, rather than employing an army of people to investigate your income every month before deciding how much to give out.

I'm not entirely certain that I've actually answered the question you intended to ask. If I'm way off the mark, could you try rephrasing a bit?