I recently realised, after a long time of not caring, that there are two ways to create a web project, with only one allowing Release Mode compilation.
Release Mode allowed: File -> New -> Project -> Visual C# -> Web -> ASP.NET Web Application
Only Debug Mode allowed: File -> New -> Website
Of course, from the command line, both of these options are available, as the options taken when creating any project are simply preset instructions for the compiler switches.
However, I feel it's important to note that only in Release Mode can the source of the website be compiled into a single DLL.