Makes sense if the emergency didn't require an immediate landing.
BOS makes it much easier to re-accomodate folks (DL hub/focus city with daily flights to CDG / AMS / LHR for re-rerouting displaced passengers), easier to bring in a new plane (if necessary), plus you remove any issues around passports and visas since the plane lands back in the US instead of Canada.
Obviously if the situation was more urgent they would've gone to one of the handful of closer airports they overflew