This is indeed a logic problem that exists in many countries that have partial legislation.

But it still helps. It removes the stigma, but most importantly: In many countries going after substance users is a low-hanging fruit to police. Instead of solving real crimes if you raid a club you can arrest a lot of "criminals", improving your statistics.

In the "war on drugs", in 99% police is going after the users, not the dealers, because the latter are far harder to catch.