Because they were there on their own time. Do you know what informants are? They're members of the group they're informing on. In criminal cases, they're almost always still committing crimes. The information they provide protects them. Likely the case here.
And the degree to which these people informed is not clear. The 26 could refer to anyone the FBI made contact with or received any information from. Trust me, the feds are well integrated into every political movement in this country. It's an open joke in left-wing circles (who are historically who the feds have targeted).
It don't think it's plausible to say that this whole thing was manufactured by the feds, which I take to be the insinuation? I think it's more likely that these people were stupid, passionate, and misled by a man they deeply adored into thinking the election was stolen and things got out of hand. As someone who was present at a lot of Pro-Palestine protests, I can attest that though people didn't show up with the intention to be aggressive, when cops show up and start shoving, all bets are off. Just human nature at that point. That's my best explanation.
And I'm willing to admit I'm wrong if more info comes to light, but what Willie posted is not that big and he misrepresented it on top of that (or at least the tweet he linked did).