documentclass[10pt,article]{amsart}

begin{document}

tableofcontents

section{Section A}
subsection{Subsection (16px Indent)}
subsection{Subsection (16px Indent)}
section{Section B}
subsection{Subsection (16px Indent)}
subsection{Subsection (16px Indent)}

end{document}

documentclass[10pt,article]{amsart}

makeatletter
def@tocline#1#2#3#4#5#6#7{relax
  ifnum #1>c@tocdepth % then omit
  else
    par addpenalty@secpenaltyaddvspace{#2}%
    begingroup hyphenpenalty@M
    @ifempty{#4}{%
      @tempdimacsname r@tocindentnumber#1endcsnamerelax
    }{%
      @tempdima#4relax
    }%
    parindentz@ leftskip#3relax advanceleftskip@tempdimarelax
    rightskip@pnumwidth plus4em parfillskip-@pnumwidth
    #5leavevmodehskip-@tempdima
      ifcase #1
       oror hskip 1em or hskip 2em else hskip 3em fi%
      #6nobreakrelax
    hfillhbox to@pnumwidth{@tocpagenum{#7}}par% <---- dotfill -> hfill
    nobreak
    endgroup
  fi}
makeatother

begin{document}

tableofcontents

section{Section A}
subsection{Subsection (16px Indent)}
subsection{Subsection (16px Indent)}
section{Section B}
subsection{Subsection (16px Indent)}
subsection{Subsection (16px Indent)}

end{document}

documentclass{article}
usepackage{amssymb,amsmath,amsthm}
begin{document}

tableofcontents

section{Section A}
subsection{Subsection (16px Indent)}
subsection{Subsection (16px Indent)}
section{Section B}
subsection{Subsection (16px Indent)}
subsection{Subsection (16px Indent)}

end{document}