Those who are not for us are against us — New Testament, Mark 9:40
There is no more neutrality in the world. You either have to be part of the solution, or you're going to be part of the problem — Eldridge Cleaver
However you word it, there are statements inferring that, by not actively addressing an issue, you are contributing to it. This is, in many cases, true. If a customer has a problem, and you do nothing, or worse, take action that has no way of benefiting the customer, you're actions are persisting the problem. If you're a white male, and don't acknowledge or address your privilege, you are persisting the problem. If you don't denounce the mistreatment of asylum seekers on Manus Island, you are persisting the problem.
So why do I have a problem with this phrase? I'm sure the intent of the statement-makers is to draw attention to the fact that people tend to stick their two cents in where it doesn't help. The problem I see is that it's being used as a way of judging people's actions (or inaction); or in a way that causes the speaker seem self-righteous and superior.
I went to a rally many years ago, and was asked by a pimply-faced youth behind the Socialist Alliance desk what I did to fight the injustice of gay rights (the cause we were all there to march for). I've never seen someone 10 years my junior look down their nose at me, not least someone who purports to represent activist for social good. Nowadays, my email inbox is full of messages from avazz.org, change.org, getup.org.au, allout.org, and more. I read half of them. I sign most of the petitions I read. I rarely donate based on requests in email. By definition, I'm part of the problem, because I'm not part of the solution for more than half of those emails.
This perspective works in a world where there is only one problem, and everyone is on one side or another. We don't live in that world though. Our lives are diverse and complex. They touch others' lives in different ways. We all experience different problems; and where we experience the same problem, we experience them in different ways.
It's time to stop using the sentiment of "for or against" as a battle axe for your cause.
The other misuse of the sentiment is as a defensive shield. One of my clients uses Slack, a group communication tool. In it, people ask questions from time to time. This morning, the following conversation occurred:
Person 1: can anyone get to http://example.com/ ?? Person 2: Yes but it is throwing up an error.
Person 1: then the answer should be "no"
Person 3: Or the question should be "is anyone getting an error"
Person 1: if yr not part of the solution yr part of the problem!
In this case, Person 2 was helpful and part of the solution. Person 3 was arguably trying to help Person 1 improve their communication skills, addressing a different problem. Person 1 was defensive and passive-aggressive, creating a new problem. It's never black and white. I don't believe you can be a part of every solution. Let me leave you with another quote, which I propose becomes a precursor to the use of any of the above phrases.
Be kind, for everyone you meet is fighting a hard battle — Ian MacLaren