There's no justification for it, people are just speculating. The BoJ buys equities because of long-term structural issues with the Japanese economy (e.g. low birthrate, low immigration, aging population) that cause deflationary pressures in all asset classes. The current situation in the US is nothing of the sort.
This is factually incorrect... I cannot believe that I'm actually making use of my polisci double major but your understanding of the word is missing YUGE parts of economic history such as: https://en.wikipedia.org/wiki/Lost_Decade_(Japan)
