What happens inside beamerpost and theorem environment

If I compile

documentclass{beamer}
%usepackage[scale=4]{beamerposter}
makeatletter
setbeamertemplate{theorem begin}
{
  {color{blue}inserttheoremheadfont% bfseries
  inserttheoremname
  ifxinserttheoremaddition@emptyelse (inserttheoremaddition)fi%
  :} normalfont
}
setbeamertemplate{theorem end}{%
 }
makeatother
begin{document}
begin{theorem}
qqq
end{theorem}
end{document}

theorembody is normal.
But, if I compile with beamerposter

documentclass{beamer}
usepackage[scale=4]{beamerposter}
makeatletter
setbeamertemplate{theorem begin}
{
  {color{blue}inserttheoremheadfont% bfseries
  inserttheoremname
  ifxinserttheoremaddition@emptyelse (inserttheoremaddition)fi%
  :} normalfont
}
setbeamertemplate{theorem end}{%
 }
makeatother
begin{document}
begin{theorem}
qqq
end{theorem}
end{document}

theorembody is italic!
Why, and how repair it?