As someone who understands little about Tether, it seems to me that why it matters is pretty obvious: since Tether can be issued by the authority that controls it arbitrarily, it enables that authority to 'print' the currency at will. This is similar to how governments that control currencies may just generate more of it. Wasn't the thought of Bitcoin to be against this very thing? Didn't the very first block of Bitcoin contain a message condemning the practice of bailing out banks by 'printing' money? It seems that some people with business-smart skills picked up on this and integrated the same principle into cryptocurrencies, and that principle is called Tether. And it appears to completely defeat the point of crypto-currencies.

But I might be wrong. I'm just a lay-person in this or whatever. Maybe I'm misunderstanding things.