I took a 1970s huffy sea pines 3 speed, unlocked, from behind a dumpster a few weeks ago. Both tires were flat but it was in otherwise great shape. I let it sit there for 24 hours and then took it under the assumption that someone was throwing it away, but didn't want to heave it into the dumpster. That bike was stolen off my porch about 2 weeks later. So, I guess that was karma if I did actually steal it, but I'm pretty sure it was garbage.

Anyway, if you're having trouble determining the ethics, then it's probably not terribly ethical or unethical either way, so I say go for it. Also, if you're going to steal a bike, may as well make it a nice one and hope it has some sweet gear in the panniers!