Someone paid him.
Considering the quality of Gronckle Iron, that would be fucking terrifying
Oh no, what I mean is that people might be turned off at the fact that, while female, Hiccup is still essentially sleeping with the Grimborn Brothers. And no matter how much you reiterate that they’re females in this story, people won’t stop seeing them as such. Which is why I suggested giving them each daughters/apprentices under their wing for Hiccup to fuck instead because the pleasure would be two fold. One, you have Hiccup fucking new girls into the harem, and two, those girls are in some way related to the Grimborn Brothers, meaning that Hiccup is getting one over on his enemies.