Here's the problem. You're on a server with a few thousand other accounts, using a system-based compiler that will attempt to use all available resources to speed up the compiling of the file, and it'd be extremely easy to dump a large c++ program into your service for compiling, which would prevent other sites from actually loading properly (due to lack of available processing power to serve them).
You might be right that it wouldn't be "constantly running," however we're for hosting websites, not for allowing others to use our processing power to compile their scripts, particularly at the risk of killing or seriously slowing service to the others on the same server.