I think that the missing context is the nature and consequences of the cash bond system in the United States. That might not be a winning issue with the American public in general but that certainly is something the left wing of the Democratic party and African-American community would be sensitive to, even without necessarily condoning the violence and destruction itself (remember that these riots stem from the killing of a black man by the police, that context is always very salient).
In most countries, pre-trial detention is supposed to be a technical way to deal with issues like flight risk or tampering with evidence, not a punishment or something you fear. Importantly, those who have been arrested have not been found guilty of anything at this point. Cash bonds add a lot of perverse incentives to the system and disproportionately impact the poor and minorities.
It's very well possible that some of those arrested did nothing illegal, will eventually be found not guilty (whether they did something or not) or end up receiving a very mild sentence. Pre-trial detention and the inability to “make bail” (pay for a cash bond) means they will be under pressure to admit to things they haven't done or see their lives teared apart (not showing up for work, which can easily lead to losing your livelihood and place of living) without even having been sentenced to anything.
In a riot situation, there is also always some concerns about the standard of evidence (even if some law enforcement agencies try to use video-surveillance both to establish the crime, identify the perpetrators and enable arrest in a less dangerous situation) and the proportionality of the response.
Now, regarding campaign strategy, treating “US citizens” as a monolithic group (presumably invoking images of some white family in the suburbs of middle America as we so often see on TV) can be highly misleading. A majority (especially the Democratic coalition) is made of many minorities, which Biden needs to string together and motivate to vote if he wants to win.
If you look at their past (voting) record, the Biden-Harris ticket is very much moderate/right-of-center, with a history of pro-business and tough-on-crime politics. That puts them in a delicate position towards the Black Lives Matter movement or the left wing of the Democratic party for example. For them, there are huge cons on both sides of the debate on defunding the police or dealing with riots. Even peaceful protests pose a problem for the campaign, they would find it difficult to fully embrace the demands of the BLM movement.
That's precisely why the campaign initially seemed to refuse to comment on the record and tries to stick to the “protesters have the right to be angry but that more violence won’t solve justice problems” line (in other words: we don't like riots either but it's not simply a problem of “bad guys” rioting for no reason). In that context, the donations could be a way to show the more politically active corners of the Internet that they care about criminal justice reform and racial inequalities issues while still maintaining some distance with the candidate himself.
It also seems entirely possible to me that the donations were made by party insiders who personally feel very strongly about the issue, without any coordination or approval at the top level. In any political party, activists will tend to have firmly held positions that do not necessarily line up with the broader public mood.
