Chess sites copy a FEN to the clipboard with the trailing fields trimmed - most often the two clocks, sometimes everything after the placement. chess.js validates strictly and rejects those outright, so they are completed here before validation rather than reported to the user as malformed.
Details
Defaults are the conservative ones: White to move, no castling rights, no en passant square, clocks at zero. Castling in particular is not inferred from where the kings and rooks sit - a FEN that omits it is not claiming those rights, and inventing them would silently change what the position means.