This is because the ACM wants you to use it this way. Publications are very strict in terms of what you can/cannot do, so my first suggestion would be to not change this. The space left on the bottom of the first column on the first page is reserved for a copyright notice:
...however, if you want to remove it, add
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\makeatletter
% \patchcmd{<cmd>}{<search>}{<replace>}{<success>}{<failure>}
\patchcmd{\maketitle}{\@copyrightspace}{}{}{}
\makeatother
to the document preamble. This patch (thanks to etoolbox
) removes \@copyrightspace
from \maketitle
. The result look like this: