Bostonians will judge Patriot's Day more harshly as it is closer to home for them than other Americans from other parts of the country. There is a deep-rooted sense of patriotism in parts of the Northeastern United States that may not be as fervent elsewhere. Not to say there is less patriotism in other parts of the country.
Patriot's Day centers around a tragedy that's just not widely known. For many people (if not most) this is their first time hearing of the events that occurred that day. It would be difficult to criticize a movie based on facts and instances that just have not been researched and reported on in so much depth as more notorious acts/events have been. Bostonians were not only present and involved, but consequently more aware, more interested, and more educated on the instance and its lasting effects. How can those so invested not be more critical?
Yes, Boston area critics are reviewing Patriots Day more harshly because the topic hits too close to home. Bostonians are not known as the most reflective people, and they often hold their city at high esteem. For example, they call Boston the "Hub of the Universe." This would keep them from being impartial.
The Boston bombings had a huge emotional impact on the people who lived through it. They might have personally been a witness to some of the events or to the news coverage and the mourning that came after the event. It's natural to want to judge the movie more harshly for this reason.