Because it allows companies extracting extreme profit from labour, paying their upper management exorbitantly and their labourers starvation wages to just keep doing that.
Edit:
There seems to be a significant misunderstanding of my post.
The question posed was "How could one understand this to be corporate welfare", in conjunction with the previous qualifier of "If the rich aren't subsidizing the program"
I'm not against UBI.
I AM against record profits. Profits are the extraction of surplus value from labour. Profits are unpaid wages.
The fact that we have an environment where a working person can not meet their basic needs while their employers take in record profits is a massive problem.
If the wealth transfer happens by way of increased wages, fine. If it happens by way of government transfers via UBI paid for by those same corporations, fine.
The premise to which I was responding was one where the wealthy were NOT the ones footing the bill.