This reason just like all the other reasons that have been used to try and strip away every shred of privacy that we have (think of the children, fight terrorism and so on) is complete bogus.
My mom was working as an admin person in a police station in France back in the 90s and from the many conversations she had with the cops, knowledge was never the problem. They already knew back then where all the drug hot spots were but truth be told there has been a lack of enforcement of the law.
Why you ask?
Simple, in most of the suburbs were drug trafficking happens, you have high unemployment, people low education and very few prospects for their future, so it has been said that the government has avoided coming down too hard on drug traffickers because they don't want these areas to erupt in chaos and start rioting.
In a way, the French government has used the drug money to indirectly buy social peace.
If the French government wanted to fight crime and get rid of these drug traffickers, it could simply legalize pot like in Germany and Canada and kill the black market.
Instead, it chooses once again to go after the sellers and the buyers. It makes zero sense because unless you are going to enforce the law by punishing the traffickers with the death penalty like in some countries, we know from all the data of the last 30 years that you cannot stop the flow of drugs.
We can't even keep drugs and cell phones out of jails/prisons, how is it that we think we can keep them out of entire countries?
The solution is simple, you legalize it and tax it and make it hard for kids to get it just like we do today with alcohol and tobacco.
That is the only sensible approach. What do you think the drug cartels fear most? The cops or legalization?