A public good is a technical term in economics. One useful way of categorizing goods is to place them along two axes - excludability and rivalry. Public goods are categorized non-excludable and non-rival. Non-excludable means you can't stop people who don't pay for the good from consuming it - like clean air. Non-rival means that one person's consumption of a good does not affect another person's. Digital goods are typically non-rival.
A cash transfer doesn't really meet these conditions - it's excludable, in that you can restrict access to citizens or to whomever. And it's rival - there are only so many dollars to go around.
That said, I think inflation is a very real concern for any basic income scheme.
