If LiveCoin is so reputable, and they knew about the MONA attack within a few hours, why did they keep trading the asset?  Why didn't they shut it down?

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.

Is LiveCoin responsible for the attack?  No, the hacker is.
Is LiveCoin accountable for continuing to trade an asset they suspected is under attack?  Yes, they are, not their clients who were holding that asset.
Is LiveCoin taking advantage of their (illegal) terms of service to shirk their responsibility to their clients?  Yes, and they admit it.

Businesses suffer losses all the time, there's inherent risk in almost every business.  The biggest risk any business faces is losing all their clients, and ironically this is directly linked to how said business treats those clients.  LiveCoin is far from a "reputable business," just look at how they use their ToS to reject any and all accountability.  Those of us who would like to promote cryptocurrency into the mainstream should be ashamed of businesses such as LiveCoin, and we certainly shouldn't be defending them.