I'm curious about the dominance of bikes in the Netherlands from a different direction.

The primary reason there's no system for handling abandoned bikes in the US is not that nobody uses bikes. Bikes aren't especially common, but the issue of an abandoned bike isn't going to come up -- ever -- because of the extreme frequency of bike theft. An abandoned bike will quickly be stolen, just like a non-abandoned bike. Bird's scooters don't have this problem because they're tracked. But the background reality of bicycles may contribute to jzl's (false) impression that you should be legally able to just take scooters that aren't locked down.

How much of an issue is bicycle theft in the Netherlands? How long would you expect to be able to own one before having it stolen?

Years ago I saw somewhere that an estimated two million bikes are stolen each year on a population of (then) fifteen million people. So that averages out to once every seven years if everyone rode bikes, but if we remove the really young and old that's probably closer to once every six years.

This does not represent my experience at all, I've had bikes stolen once every three years on average. It won't surprise you that students learn to buy very cheap second-hand bikes with very big bulky locks really quickly here.

On the plus side, I've never been worried about junkies mugging me. They steal/sell bikes for drug money instead, and I honestly wonder if that is a uniquely Dutch thing.

On the west coast, bike theft is common. Junkies and/or homeless people steal them and sell them for parts. Or change parts around and paint them so they can't be identified.

Also, there's supposed to be organized rings where bikes stolen in Portland are trucked to Seattle (so they're much harder to identify as stolen) and vice-versa.

Don't the scooters generally get rounded up to recharge every couple days? That sounds like a good system but I don't understand how it's relevant.

I wouldn't know, I'm not in a place where Bird operates. But it was just an example - if it is like you say, it sounds like a different type of system should be put in place instead (probably some form of licensing).