By Stallman's own telling a free Objective-C frontend was an unexpected outcome. Until it came up in practice he thought a proprietary compiler frontend would be legal (https://gitlab.com/gnu-clisp/clisp/blob/dd313099db351c90431c...). So his stance in this email is a reaction to specific incidents, not careful forethought.

And the harms of permissive licensing for compiler frontends seem pretty underwhelming. After Apple moved to LLVM it largely kept releasing free compiler frontends. (But maybe I'd think differently if I e.g. understood GNAT's licensing better.)