Uber Pool in Boston suffers in certain areas from many one-way streets and riders who call at the end of a dead-end street rather than walk 50 meters to a main street. Uber has added a mandatory walk-to-pickup beta feature to Uber Pool in specific neighborhoods in Boston starting January 2017. Previously, Uber Pool would suggest better pickup spots, but they were optional.
The Boston Uber Pool mandatory walk to pickup spot neighborhoods are: Back Bay, Fenway and Harvard Square. I could certainly think of some other neighborhoods needing this feature, so Uber must have used some demographic/statistical measure to pick on these three neighborhoods first.
In these zones, you will be asked to walk up to 250 meters (1/6 mile)–roughly like Commonwealth Ave. to Boylston St. in Back Bay, but more commonly like one east-west block in the Back Bay area.
You can zoom in and pan on this map of Uber Pool walk to pickup zones.