This is the most succinct explanation of the reality of translation that I’ve come across.I mean, when you translate such a dative with "from", that's more like a concession to the English language than saying anything about the word's function or supposed equivalency with something else in the original language.
It looks like habrá de asustar is an haber+de+inf. construction which means obligation, but I've never seen that with a future tense haber. In English a literal translation sounds too wordy, and a bit strange: "Why will one be obliged to fear a hat?". But the main question here is why the future tense instead of another, like present or even conditional?Mostré mi obra maestra a las personas grandes y les pregunté si mi dibujo les asustaba. Me contestaron: “¿Por qué habrá de asustar un sombrero?” -El Principito, I
J'ai montré mon chef-d'oeuvre aux grandes personnes et je leur ai demandé si mon dessin leur faisait peur. Elles m'ont répondu: "Pourquoi un chapeau ferait-il peur?"
Me contestaron: “¿Por qué habrá de asustar un sombrero?” -El Principito, I
So that's a bit complex but it looks like what the grammar describes as a Mexican variant of the Suppositional Future in a question form: "I wonder why one would fear a hat."14.6.3 Suppositional Future
An important function of the future tense in ordinary Spanish, especially in Europe, is to express suppositions or approximations... In questions, the future expresses wonder, incredulity or conjecture..."Que sera esto?"- "I wonder what this is?"... Kany, 190, notes that this use of the future is much more common in Spain than in Latin America, where deber (de) [plus infinitive] often replaces it... In Mexico haber de is commonly used for deber de in this construction.