beforeskip does not work with tabu

documentclass[paper=a4, pagesize, DIV=calc, version=last,fontsize=8pt]{scrartcl}
usepackage{lipsum}
usepackage{showframe}
RedeclareSectionCommand[beforeskip=-1cm, afterskip=.2cm]{section}
begin{document}
section{One}
lipsum[1][1]
section{Two}
lipsum[1][1]
section{Three}
lipsum[1][1]
end{document}

produces following output:

This is correct and expected. But when using tabu (I am using beforeskip=-10cm on purpose):

documentclass[paper=a4, pagesize, DIV=calc, version=last,fontsize=8pt]{scrartcl}
usepackage{longtable,tabu}
usepackage{showframe}
RedeclareSectionCommand[beforeskip=-10cm, afterskip=.2cm]{section}
begin{document}
section{One}
begin{longtabu} {l}
    A 
end{longtabu}
section{Two}
begin{longtabu} {l}
    B 
end{longtabu}
section{Three}
begin{longtabu} {l}
    C 
end{longtabu}
end{document}

I am getting following output:

This is not expected: the spacing before section is not 10cm. Did I misunderstand something?