I see your point, but, I wouldn't have done that without trying to find out the owner first.

I know people who have adopted abandoned bikes on campus...They left a note on the bike for weeks with their name and number, saying they were interested. Simultaneously, they posted on multiple campus forums (craigslist or flyers could substitute) And after a month of the bike not budging and no calls, they took it. And posted up that they had it if the owner wanted it back, with contact info.

I think that kind of "theft" if you want to call it that is perfectly okay.