This is not like an airport, in which people are screened for security purposes when they enter the terminal. The random inspection of handbags doesn't just apply to convention attendees in Boston -- it's being done to anyone who takes an MBTA train to the vicinity of the convention site.
If there is a legitimate reason to believe that there are major security concerns at a large gathering like this, the solution is not a large-scale abrogation of the Fourth Amendment . . . instead, they should just cancel the event, or hold it someplace where the general public will not be bothered.
It is an infringement of civil liberties alright, if your purpose is not to attend the convention.