They might be looking for drugs indeed, also people who are either staying illegally (overstayed their visa, entered with someone else's passport and have no visa or permit, living and working in a country with a permit from another country), or asylum seekers who applied to have their status recognized in one country but would rather live in another one.
I could write a lot about the legalities and politics of it but that would be a bit off topic here. As a practical matter, that's the reality of the Schengen area since circa 2015: common visa area, no visible border facilities and open road traffic, minimal formalities for air traffic, ostensibly random checks targetting cheap public transportation and the poor generally, almost systematic checks on some borders (e.g. between France and Italy).
If my experience is anything to go by, all those checks have become more (and not less) frequent over the years. Whether they are legally permitted or whether France is stretching the rules is a matter of interpretation but it's a fact that the original implementation of the Schengen area did make for even more open borders.