The tendency of online US leftists to refuse solidarity with social movements on account of their secondary characteristics, e.g., the gilets jaunes (yellow vests) on account of 1/3 of them being racists, is a mode of expression of the consumer model of activism.
This model views social movements not as malleable, living entities, which through participation can be reworked and redefined. Rather it sees them as fixed objects, against which the "activist" defines themselves.
Occurred to me after listening to the Aufhebunga Bunga analysis of gilets jaunes, which is recommendable. They characterize the movement as centrally a movement of the rural poor against anti-poor policy. They point out that it could easily turn left or right, and that for the left to forsake the movement would be to hand it to the right.