The capitalization of the first noun is not mandatory. It is capitalized when using the traditional conventions but publishers can choose to use simplified ones where only the first word is required to be capitalized (here Le).
Note that this non capitalization can be found elsewhere, for example in this book:
Sometimes the rules are broken and fully lowercase titles are used:
One reason the lowercase variant might has been selected in the Pour les nuls collection is that it removes a possible ambiguity, le Français sans faute pour les nuls can be understood by a careless reader as The Frenchman without misconduct for dummies.
Same with The Englishman for dummies