Images that are adopted as Unicode glyphs (including emoji) have to be in the public domain, i.e. not copyrighted or trademarked. When they were assembling the Unicode codepage for characters from vintage computer character sets, they left out the Atari ST Dobbshead character because the intellectual property status of the piece of vintage advertising clip-art it was derived from was unclear. Presumably someone (Prince’s estate and/or Warner Music, I’d guess) owns the copyright and/or has a trademark claim on the Prince symbol