I'm not sure if this played into the researchers decisions, but giving a small but meaningful amount of money to very low income people allows you to increase the size and duration of a study for a given level of research funding. many low wage workers live on a dollar a day or less, I think wages for textile workers in Vietnam for ex are $2 a day, and poverty levels in Africa are even lower

If instead they gave $40k / year to people in the US, they could only fund 625 person-years. So this design would limit the ability to predict how UBI would work in the US, but 1) provides help to those who need it most and 2) maximizes your sample size