Because the majority of hentai is drawn/commissioned by straight men who feel so entitled to their porn they don't give a damn if it's rape or the characters depicted are underage.

And Celica IS underage--she's 17 during Echoes and since all pirate rape is depicted as taking place within days/weeks of her having SOMEHOW lost to them, technically every illustration of her as a pirate sex slave is child porn, which includes this one. (And this extends to ones involving Mae, also 17, and Genny, the youngest unit on Celica's route at 15.)

Sad thing is knowing that Celica, Mae, and Genny all canonically get married after the game to guys who are, you know, decent human beings, so the consenting sex scenario is right there. Depictions of rape, unfortunately, have been normalized to the point that it's treated as an actual kink, so the idea of them being raped by pirates is legitimately pleasing to imagine for many people.