I understand what you say but put it this way, if a service is open to misuse by the end user then the owners of mixers are on the inevitable road to being arrested for money laundering. Having a maximum limit per mix would only allow those who wish to misuse the service to circumvent the rules with relative ease.
The only way to avoid that would be from them to retain customer data including KYC which they can pass on to relevant law enforcement agencies when legally required to do so but that is counterproductive to those seeking anonymity when mixing and not many would be willing to hand over their details to mixers.
One option for a way out of mixer owners to avoid prosecution would probably be for them to implement KYC and though it defeats the primary privacy objective, there is a logic behind it. If any form of KYC were to be implemented by a mixer then the amount of trust being given to them by their clients would be immense and basically a huge put-off.
Maybe you don't even need to go that route. They can only create a value limit per mix.
In almost all countries, there is a maximum amount that is not considered a bribe. So if this value is not exceeded in each mix, there probably won't be any big problems.
Of course, I don't know all the legal criteria that can be followed. But, the owners of these sites, have to study more the laws of the countries where they operate, to try to protect themselves.