minority interest using acquisition method

Assuming firm A buys 80% of firm B, after consolidation, why is the minority interest considered equity? In other words, we combine B’s assets and liabilities with A’s assets and liabilities, but the minority interest is shows on A’s balance sheet as ‘minority interest’ under equity.

Isn’t this minority interest controlled by outside investors? Why would those holdings by outside investors go on A’s balance sheet?

Thnx!

Minority interest is a deduction of total equity in consolidated BS in percentage of equity not controlled by parent.