The study by Nicole Maalste of Tilburg University found most local authorities don't enforce the ban even though they have incorporated it into local laws, DutchNews.nl reported Wednesday.
Councils did not enforce the ban because the sale of marijuana to non-residents had not created any problems or because few foreigners visited the towns, Maalste said local authorities told her.
She found non-residents were excluded from shops in 23 of 103 local council areas, primarily in the country's southern regions, with licensed marijuana cafes.
The Hague adopted the law to discourage drug tourism in the border areas. Dutch lawmakers are considering changes in the law to allow local councils to experiment with legal production of marijuana.