Nested itemize environment affects vertical spacing of parent itemize environment

The package enumitem somehow solves the problem. Note that the package redefines beamer list features.

\documentclass{beamer}

\usepackage{enumitem}

% redefine default beamer item labels
\setitemize{label=\usebeamerfont*{itemize item}%
  \usebeamercolor[fg]{itemize item}
  \usebeamertemplate{itemize item}}

\begin{document}
  \begin{frame}
    \begin{itemize}
      \item a
      \item b
      \item c
      \begin{itemize}
        \item d
        \item e
      \end{itemize}
    \end{itemize}
  \end{frame}
\end{document}

This issue appears to be solved by leaving a blank line before the nested itemize:

\documentclass{beamer}

\begin{document}
  \begin{frame}
    \begin{itemize}
      \item a
      \item b
      \item c

      \begin{itemize}
        \item d
        \item e
      \end{itemize}
    \end{itemize}
  \end{frame}
\end{document}