Do you think it is that hard to automatically detect _possible_ pieces of code in a post and to inform the poster that he may want to revise the contents as an additional step before the data is actually placed online? This may have been proposed before but it seems to me like a good idea, given the number of people who don't care and don't read instructions, among others...
But the problem is not the difficulty of doing it correctly; the problem is with people who don't even think about doing it at all - so having a button wouldn't help, because they still wouldn't press it!
:-(
Hence the original request to auto-detect what looks like source code and prompt the unobservant user, "don't you think you should have formatted this as source code - like this...?"