In VS2012 Website publishing, symbols are always excluded by default. Why? It comes down to a design issue where website projects don't actually have a build configuration. If you look at the configuration manager in VS when your solution is in Release mode, the website project will always be in Debug mode; there are no other options. This is because website projects are not MSBuild based, and hence do not respect MSBuild configurations.
Instead, you can edit your .pubxml to tell it to include the symbols. Try adding this:
<PropertyGroup>
<ExcludeGeneratedDebugSymbol>False</ExcludeGeneratedDebugSymbol>
</PropertyGroup>