It is a legal issue that is not specific to Coinbase or Bitcoin.
Here is an article that explains it:
https://ftalphaville.ft.com/2015/03/24/2122678/bitcoins-lien-problem/In general, if someone uses an asset for collateral on a loan and they sell the asset and default on the loan, the lender can legally force the buyer to give them the asset. Bitcoin is not immune to this law. So, if someone using their bitcoins as collateral sells them to Coinbase and Coinbase sells them to you, you could be forced by the law to to give the lender your bitcoins.
Stolen bitcoins have the same issue. If you inadvertently or indirectly buy stolen bitcoins, you could potentially be forced to return them.
My own opinion is that it could get messy for a couple reasons. There are no actual bitcoins, and bitcoins are quite fungible. Who can say who got the bitcoins? Also, if you deposit them in another exchange, wouldn't that instantly make it someone else's problem?