Typically quantity is limited at safe sites. You can use, but you don't get to blow a whole 8 ball in one go.

I've paid more money to drink alcohol in a location I prefer countless times, so I'm sure some paid for the privilege of doing their drugs somewhere more interesting than the clinic.

Same reason you'd go to a bar to have a few drinks rather than pick up a six pack on the way home. There's more to these things than the substances themselves, something silver bullet salesman often forget to point out.

That's the theory and it's been shown to work to some extent. The trouble is the drug problem is very complex and essentially intractable.

The best we can do is to aim for harm minimization whilst understanding that the minimum will never be a deep notch but will always be shallow.