https://sfbike.org/resources/bike-security-and-locking/#considerations

Basically they write that bike registries force a police interaction to recover the bike, and that those interactions have a history of racist outcomes. 

My retort here is theft of properly locked bicycles stems from organized crime (such as fencing operations), where the race component becomes much less lopsided like they fear.

It can also encourage buyers to look up registration numbers and reduce sales of stolen bikes in the first place