Reputable exchanges don't keep trading assets they suspect are under 51% DDoS attacks. Coinbase shuts down assets at the slightest inclining of an attack. It pisses off their customers to no end, but guess what, nobody loses hundreds of thousands of dollars
This doesn't make them scammers
Yes it does....
They are selling an asset to someone and the buyer will never be able to take possession of said asset because the seller is refusing to deliver.
This would not be unlike if I were to sell a gold coin that I do not have and have no intention of delivering to any buyer (I would never do that).