Please see my response to a similar comment below:
(Good question (there are several comments regarding how curb extensions affect bike lanes). It would be irrational to create bike lanes only to abruptly truncate them at intersections, forcing bikers into car traffic.
By design, bulb-outs don't encroach on the bike lane at all (and thus, don't 'narrow' the portion of road used for movement), but instead only occupy the width of the parking lane at the corner. Because parked cars don't typically wrap around the corner, cyclists naturally 'cut' these corners diagonally--but then they're still forced to merge adjacent to fast-moving cars that haven't confronted any slowing measures across the intersection.
With this in mind, we designed our bulb outs to accomplish what a curb-height, rounded bulb-out might not--slowing traffic not just on the turn, but across the entire intersection zone. Their bulk and height, in addition to ample surface hatching, reduce vehicular speed from all street directions, and the chamfered geometry still allows cyclists to 'cut corners' without being forced into traffic in the intersection.
In locations where bike lanes have been created by eliminating parking lanes, I agree that curb extensions are problematic. Our in-depth proposal (not fully described in this article) allows for site-specific variations, which in a case like this might have curb extensions only on one side of the street (eg. the new bike lanes on Oak and Fell Streets).)