Unless the bike was truly "abandoned" in which case the original owner gave up legal ownership of the bike. If I leave a couch on the curb and someone takes it before the garbage man comes, thats not technically stealing. I admit, the question is more difficult when it comes to a bike locked to something because you have no objective evidence of the owner's intent to abandon it. Legally it presents a more difficult problem. Ethically, i think you could reasonably conclude that dude didnt want his grips anymore and they were free for the taking.

For a real ethical mind bender here is one. Dude leaves his bike on a rack. Goes to a bar, gets drunk and hit by a car, and is in a coma for 2 months. While he is in the coma someone puts a sign on the bike that says free bike parts. Assuming you do not have knowledge of any of the facts pertaining to the owner's current situation and simply come upon the bike, is it ethical for you to take the seat?

A. Yes, because I didnt know the guy was in a coma and i read the sign and though the owner gave it away.

B. No, regardless of whether yoyuknew about the owner's situation or thought you were stealing or even had a reasonable basis for your subjective belief, objectively you still took property belonging to another person