Unless it's a limited access highway bikes are allowed to use the whole road. This bike lane does not safely guide cyclists through the left turn. It forces cyclists to wait until a very short interval, during which motor vehicles run the red light and block the intersection, meaning cyclists are forced to wait through multiple, lengthy cycles to safely make the turn.

Or they can just use the road like they're legally allowed to do.

If motor vehicle operators would like to start obeying the rules of the road, such as not running red lights, then I'm certain most cyclists would happily consider taking the bike lane here. If you want to educate, I recommend you start with the folks driving multi-ton death machines before the folks riding 20 lbs bicycles.

While this may not be the recommended approach, I believe it is perfectly legal. https://malegislature.gov/Laws/GeneralLaws/PartI/TitleXIV/Chapter85/Section11b does not appear to prohibit it. I suspect that the "jug-handle" is easier and safer for less experienced cyclists, but that the traffic signal's cycle makes it annoying for experienced daily commuters.

JW is correct.

The dangerous traffic situation at Porter Square is not fixed. I didn't report this to find out if the bicyclist had legal rights to use traffic lane and not the bicycle lane.
It may be perfectly legal but that shouldn't prevent City from trying to make that intersection safer for everyone. Just remember what happened on Oct. 5 when an experienced bicyclist was killed in collision with 18-wheeler. Could this tragic accident have been prevented had he used the bike lane? If the bike lane was put in after the accident to prevent future tragedies, shouldn't City try to educate bicyclist to use the bike lane at this very dangerous intersection even if it is legal not to?
If the cycle time for the bicyclist is too short as Ian pointed out, shouldn't City conduct a study to come up with more optimal cycle time - the bicyclist should have at least the same time as pedestrians to cross.
The City should set up a visible traffic enforcement team at the intersection during rush hours to educate everyone involved - the motorist running red lights, the pedestrians jaywalking and the bicyclists weaving in and out of traffic dangerously and/or not obeying traffic laws.
We have to make streets safe for everyone.

As mentioned above, cyclists can choose to take the left turn from Mass Ave to Somerville Ave from the travel lane or via the jug handle. We are planning changes to this intersection, including signal work and other small physical changes that will simplify the operation and lessen the time cyclists must wait to use the jug handle. We hope this will make the use of the jug handle turn more attractive to cyclists.