There are twisted incentives, and the FBI and police have been known to send low level offenders into dangerous situations on threat of extreme jail sentences, but that's not what happened here.
The proud boys are a domestic terrorist organization, and always have been. Their membership handbook lays out how to advance in the organization, and one of the steps is to go out and get into a fight with people who the proud boys consider "enemies". Just go out into the streets and bodily attack someone, and possibly get arrested for doing so.
I guess the final step is to turn informant and squeal like a stuck piggy.