Found an iPhone but can't activate?

I found an iPhone 4 in the park and I factory reset it, but the original owner has linked it with their Apple ID, meaning I can't activate the phone.

You should return the phone to the police. Many people lose their phone and it really devastates them. Perhaps you should do the right thing in this case.

Most likely owner will turn phone to lost mode and send message to phones screen with his phone number. Call him back and make him happy.

Exactly!

that is the security feature that Apple put in the latest OS s
It is to make it not worthwhile to steal a phone… And make it safer to have one.

if you take it to a network or AppleStore, they can look up the real owner and get it back to them (reward?) etc.

but you, no you can't use it
even if you could, if the owner reports it stolen or lost, the network will black list it - won't work on any network (worldwide)

Set a good example to your kids, return the phone and you will be happier.

Lets all hope that you lose more than a phone in the near future and then have a think.