Taking a bike that is left on the curb without the owner's permission can be considered stealing, as it would be taking someone else's property without their consent. In most jurisdictions, property left on the curb is still considered the owner's possession unless it has been explicitly abandoned or discarded.
If you come across a bike on the curb and you are interested in it, it is best to assume that it belongs to someone and to make an effort to locate the owner or notify the appropriate authorities, such as the local police department or municipal services. They can assist in determining the rightful owner or guide you on the proper course of action.
Laws and regulations regarding abandoned or unclaimed property can vary depending on your jurisdiction, so it's essential to familiarize yourself with the specific laws in your area to ensure you act within the legal boundaries.