Why is the spacing in this itemization so bad and why can it be fixed by adding a percentage sign

The definition of itemize in beamer is buggy. The problem is that is sets a color before the list, and color commands inserts whatsits and this has the effect that the space suddenly matters. beamer should at least add an \unskip before the color:

\documentclass{beamer}
\makeatletter

\renewcommand{\itemize}[1][]{%
  \ifhmode\unskip\fi %<<< or \unskip \par
  \ifblank{#1}{}{\def\beamer@defaultospec{#1}}%
  \ifnum \@itemdepth >2\relax\@toodeep\else
    \advance\@itemdepth\@ne
    \beamer@computepref\@itemdepth% sets \beameritemnestingprefix
    \usebeamerfont{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamercolor[fg]{itemize/enumerate \beameritemnestingprefix body}% problem
    \usebeamertemplate{itemize/enumerate \beameritemnestingprefix body begin}%
    \list
      {\usebeamertemplate{itemize \beameritemnestingprefix item}}
      {\def\makelabel##1{%
          {%
            \hss\llap{{%
                \usebeamerfont*{itemize \beameritemnestingprefix item}%
                \usebeamercolor[fg]{itemize \beameritemnestingprefix item}##1}}%
          }%
        }%
      }
  \fi%
  \beamer@cramped%
  \raggedright%
  \beamer@firstlineitemizeunskip%
}

\begin{document}
    \begin{frame}
        \begin{minipage}{0.5\textwidth}
                \begin{itemize}
                    \item A. (requires three sub-items)                   
                    \begin{itemize}
                        \item A.1
                        \item A.2
                        \item A.3
                    \end{itemize}
                    \item B
                \end{itemize}
        \end{minipage}
\end{frame}
\end{document}

Thanks, I found another reason for not relying on indentation but rather on spacing out the input. ;-)

Yes, itemize in beamer has a small bug, but it becomes irrelevant if you type in like the left column below.

\documentclass{beamer}
\begin{document}

\begin{frame}

\begin{columns}
\begin{column}{0.5\textwidth}   

\begin{itemize} 
\item A. (requires three sub-items)

\begin{itemize}
\item A.1
\item A.2
\item A.3
\end{itemize}

\item B

\end{itemize}
\end{column}

\begin{column}{0.5\textwidth}   

\begin{itemize} 
\item A. (requires three sub-items)
\begin{itemize}
\item A.1
\item A.2
\item A.3
\end{itemize}

\item B

\end{itemize}
\end{column}

\end{columns}

\end{frame}

\end{document}