That's the problem - you can recreate the binary from the source tarball. Files generated from a bison grammar *are* source. As it's generated source it's not as useful for maintenance and feature enhancement as the original grammar file that's input into bison.
So here's the low down on the process - bison grammer is input into bison (usually a .yy file IIRC), which outputs a file in C, which is then used when emacs is compiled. When you run the compilation, if the original grammar is present it'll generate the C file, and then build emacs. If the generated file is present and up to date it'll won't try to generate from the .yy because it already has the generated file. So if the .yy is not included, this goes completely unnoticed as one can build from source.