I would be hesitant to get into the knife/gun comparisons.
The charge isn't the anonymization of the money, it's specifically the concealment of money produced by criminal activity, and whether or not that's something that is allowed based on NL law is really the question, as is the motive of the developer/service providers.
This next part is from a US perspective, but remember that there are multiple aspects to law besides just the actual act. There has to be a motive as well.
The reason as I understand it that knife/gun manufacturers aren't really held responsible is because (arguably) their goal is not for persons to commit illegal acts.† Thus the illegal act is an exception and independent of the intention of why the product is produced, and there is not a motivation to empower illegal activity from the manufacturers.
With Tornado cash, it becomes a bit murkier I think and I suppose this is why it's being sent for examination as opposed to outright finding the person guilty. I would imagine what the judge wants to find out are things like:
1. Who was the primary audience/user for Tornado Cash (TC)? Not generalized, but who was actually using it?
2. Were there communications between the team behind TC and other entities that can be identified or no?
3. Did the TC team have awareness of who their main customers were and where the coins mainly came from?
4. Was there any campaigning by the TC team that can be found which shows they were specifically catering to people doing illegal activities?
5. Likely, a court and FIOD would want to investigate if any regional activity can be tied to Tornado Cash††, and if a known sanction region was utilizing the service, were actions taken to prevent this.
I understand that the goals of cryptocoins and the goals of Governments are opposed by design, and likely there will be constant conflicts like this for a long time with cryptocoins and governments; one wants to circumvent monetary rule, the other imposes the monetary rule. I have no personal judgement on TC or cryptocoins, but the court decisions will be interesting to read.
† - I do realize that this line blurs a lot depending on the type of knife being sold, and even worse with gun manufacturers. Unironically, the Borat movies (I forget which one) show this pretty well when Borat asks which gun is best for "stopping Jews", and the gun owner doesn't blink. Gun manufacturers I would suggest walk a fine line in their advertising, as do proponents of gun rights. I know responsible gun owners so I'm not here to case a wide net on all things gun related, but my take on a lot of weapons advertising is that it sells a violence fantasy.
†† I'm not as familiar with ETH or even how probable it is that they can find who used a service, but it's something that the teams will try to figure out. Whether or not this is a good idea long term is not the point I want to make, it's more that I think this is something governments will be interested in. Very likely, there is a vested from these governments in ensuring specific sanctioned countries cannot use cryptocoins to circumvent sanctions. I don't really agree with this ultimately, but it is important to understand the entire thought process beyond just "governments hate cryptocoins".