What is interesting is a few years earlier they could legally have sex. Then for a couple of years it's a felony. Then it's legal again.
Incorrect, barring any recent legal changes in the State of New Hampshire.
Felonious Sexual Assault: II. Engages in sexual penetration with a person, other than his legal spouse, who is 13 years of age or older and under 16 years of age where the age difference between the actor and the other person is 4 years or more; or
If he was 17 and her 15, that's only two years, well within NH's 4 year 'R&J' exemption. Indeed, by the way the statute is constructed, once legal they're always legal.
Though the second article says 18 and 15, but even at 3 years and change it shouldn't have triggered statutory rape charges by the letter of NH law.
Lacking statutory rape, they'd have go go for 'actual' rape charges, IE it was against her will, and browsing news articles, that's what they did. They simply failed to make that case.