To the people worried about fungibility, they need to remember that this blacklist via tagging is not centrally enforceable. It is an opt-in tool that exchanges can use to avoid stolen XEM at THEIR discretion.
If coins may be blacklisted, they are not fungible. It has nothing to do with centralization, opt-in, or discretion.
There mere fact it is possible for anyone to blacklist coins proves the coins are not fungible.
Truly fungible coins cannot be blacklisted. Look into the Monero literature for more details.
The ignorance displayed in your post is beyond believe. Let me try to clear this up for anyone that is about to believe this.
What they are doing is not actually blacklisting funds. They are writing a service that will automatically send a mosaic, which is what nem's on_chain assets are called, to every account that receives funds from the hacker. All that does is mark the account for other people so THEY can decide whether or not to accept those funds. It's basically like me saying I won't accept iCREBEAKERSs shitty monero because he's spreading FUD. That is my right and it has nothing to do with central authority.
All they are doing is helping others identify hacked funds, that's it. It is up to everyone to decide whether or not to accept them.
They are also not using any central authority to issue said mosaics. Anyone could write a similar service and do the exact same thing and it would again, be up to everyone to accept or not accept those funds.
So go spread your FUD somewhere else.