This simply makes clear what has been a point of confusion for travelers to/from states which have laws which in some form suggest that the possession of marijuana may be legal (with or without prescription). It remains a violation of federal law throughout the US.

Because marijuana is contraband, an Officer finding marijuana have an obligation to report that to law enforcement for whatever further action law enforcement chooses to take. That obligation is on the Officer, whether he has law enforcement authority or not. Would be the same for any federal employee who comes upon a violation of federal law.

There are many people who live in jurisdictions where marijuana possession laws have not been enforced for many years even before the current legal changes. They go on a trip to a location where it's taken very seriously and get bad surprises.

This kind of warning is helpful to those people. A way of saying, "leave it at home..."