Anyone who would leave all their pennies on the ground… is someone who does not have any sense.
Based on a Psych experiment I have read about if you put a picture of eyes next to the pile (just eyes) less people would be willing to take from it.
If** Skald **was around there’d be a slapbot heading your way.
Not necessarily. Things that are abandoned are unowned. That pile of coins looked abandoned, IMO.
If you have abandoned your bicycle, then yes it is OK for someone to take it.
$400 worth of coins are not generally abandoned. Neither are bicycles in good condition. It takes a little bit more context to determine if something is abandoned than simply seeing it lying on the ground.
$400 worth of coins are not generally accidentally dropped and then not noticed and/or left there on purpose unless they are abandoned. In this case, IMO, there’s plenty of context to determine the likelihood of $400 in the form of 15,000 coins having been abandoned.
Do you think the people who didn’t take any money thought “no, I better not; that’s someone’s money and they’ll be back for it”?
Weird anecdote: I live in a rural area, and at the far end of a longish (maybe around 500 feet) unpaved private driveway. One day years back a bicycle (in good condition) is abandoned around 100 feet or so up my driveway. I leave it lying there 9 days to see if someone comes back for it. After that, I call the police to pick it up. The officer that picked it up told me if nobody called claiming it in x number of days, it was mine (and if I didn’t want it it would be sold at a police auction.) The next day, another bike was abandoned on the same spot on my driveway. I called the police for it, too. I never called the police back because I didn’t want the damn things, so I never learned what up wid that.
It’s Theft by Finding in English law. In the US it varies but based on the amounts and circumstances you may be required to attempt to find the owner of found property.
No, I think they thought they could grab the money and run.
In this case, the $400 worth of coins were abandoned. And unless you can point to some other cases where $400 worth of coins was left in public and not abandoned, then I have to say that your claim about what generally happens fails. The only instance of a large amount of small-value coins being left in public that I know of was a case of abandonment. 
Sure, they were abandoned as part of an art project, to see what would happen. But they were still abandoned.
So: no theft.
It’s possible that you can argue that the people who took the coins should have thought they were stealing. But it turns out they weren’t.
And I think the context supports the people who took the coins. While someone might have traveled somewhere by bicycle and left it unlocked by necessity, expecting to return to it, I can’t come up with any reasonable explanation for hundreds of dollars in pennies left in a pile on the ground other than “someone left it here on purpose to see what would happen”.