Because the benefits of automation are highly capital intensive, concentrating wealth in ways that are bad. Taxing this make the benefits for those with capital less, but it probably still means automation is a great investment.
This tax just prices in the negative externalities of job-loss into automation. Job-loss really is a drag on an economy (you need consumers to keep an economy going). Moreover job-loss causes poverty, and poverty is bad in and of itself. UBI should make poverty less bad. Funding that partially by automation makes sense if that automation is partially the cause of the poverty (through job loss).
This tax just prices in the negative externalities of job-loss into automation. Job-loss really is a drag on an economy (you need consumers to keep an economy going). Moreover job-loss causes poverty, and poverty is bad in and of itself. UBI should make poverty less bad. Funding that partially by automation makes sense if that automation is partially the cause of the poverty (through job loss).