There's another fairly obvious reason why crime rates drop when you legalize something.
Yes, I know that it doesn't account for the entire difference, but I don't think legalizing marijuana is what led to the Netherlands having a lower crime rate, especially when other European nations have comparable rates (as far as I know) without this particular legislation. It's certainly better to compare to its counterparts than a nation that has been fairly...divergent, shall we say, from Europe in societal change.
Besides, I'd sooner link America's relatively high crime rates primarily to its appalling literacy rates for a First World nation, but that's my own biases being given free reign.
