Dude it's not stealing by any definition. You can just walk up and take it. While anyone can leave their bike locked to a meter or lamp post legally in most cities it's not allowed to remain there indefinately. Most cities also can't afford to come around more than once a year or every 2 years to clean the drek up. As you describe it it's abandoned, take it, it's yours. You're removing an eyesore, saving the city money and providing another parking space for someone who does ride. Reclaiming abondoned property not regulated by a licensing body (you can't just take cars or boats or vacant lots you find abandoned) is as legal as bending over to pick up a discarded half empty bag of chips to deposit into the trash. Once you've picked them up if you decide to eat them instead, knock yourself out.