The precompiled note is because Pandoc is written in Haskell for ghc; the Haskell toolchain is far from being universally available (as, say, C toolchains using gcc are) and is somewhat immature on Mac and Windows. The Pandoc source is available, of course, but one may legitimately not wish to install everything necessary to compile the source oneself. (If Pandoc were written in C, then I might not bother specifying that there are trustworthy binaries available.) There's a reason not everyone uses source-based distros like Gentoo or Arch.