People wanted to export the AST and other stuff from GCC so they could use it in other tools (e.g. syntax highlighter, code search, etc.). But by including that

People wanted to export the AST and other stuff from GCC so they could use it in other tools (e.g. syntax highlighter, code search, etc.). But by including that functionality GCC would open itself up to be used by proprietary tools, as the GPL doesn't protect the output of a program, it only protects the program code itself, so RMS was against it. It's somewhat similar to the reasoning in why Linux doesn't have a stable driver API, offering one would remove the need to include drivers in the kernel and putting them under the GPL.