If they think he's another Seabrook waiting to happen, it's the right move. And let's face it, bigger D are more likely to be a Seabrook in their 30s than a Pronger. 3 years from now letting Pietrangelo walk could seem like a genius move.
Would you have been upset if the Hawks let Seabrook walk in 2016 or traded him coming off of a cup in 2015? Of course not, everyone knew that was bad the moment that it was signed. Pietrangelo will be 30 this year so it's not a crazy move. It's more likely than not they got his best years.