Why do my SWF files contain my source code?

Hi,
I have been developing AS2 and AS3 games and such and only the last week I found out that my SWF files contain all of my source code with ALL of my variable names.
And not only that I found out that adding a single space( ) adds 2 bytes to the size of my SWF file.

So I would like to know if any of you know why the SWF files contain the source code with variable names and why adding a single space changes the size of my SWF file.

I thought my source is compiled but just compressing my source code is not called “compiling”.

Thanks.

PS: I know there are tools to obfuscate my source but why do I need them at all?