I can't think of a reason why you would want a jury trial, and the potential criminal charges that come along with it, over a reduction to a civil infraction, since there is so little risk of anything severe with a civil infraction other than a fine.
This event was not the only Black Lives Matter protest to occur in Massachusetts around that time. In at least two other communities, demonstrators blocked off other roads. The treatment of the demonstrators has varied between the communities.
By seeking to address the original criminal charges against them, the demonstrators in Worcester (this case) may be seeking to effect greater change upon the system. Conceptually if the demonstrators still faced criminal proceedings, lost the case, appealed, and then won on first amendment grounds on appeal that would affect the proceedings in other communities as well as in future cases.
To put it more generally, if you're charged with a crime under a law you see as unconstitutional or otherwise impermissible, you have the right to protest that law and challenge its constitutionality on appeal. Not just facially, but as applied. But if you are never charged with that crime under the law then you cannot bring an as applied challenge because it was never applied to you. That leaves one with the option of a facial challenge, obviously more difficult to pursue.
In the light of separate but organized protests in other areas, removing the criminal charges from one group also removes that group's ability to assist the others through a resulting appeal.
Then of course there are also collateral concerns of increased and continued news coverage and the like as the cases travel through the appeals system.
While that strategy is not a great criminal defense strategy, it doesn't seem a wholly invalid civil rights litigation strategy.
Of course all of that presumes that there is a strategy here that is primarily geared towards effecting change in the law rather than utilizing the courtroom as a bully pulpit.
