I believe they have created additional corporate entities to receive money through so they have been able to receive (and send) USD via the banking system. I understand they are primarily catering to institutional customers so they may not be willing to deal in USD via the banking system for smaller amounts.
I agree with your statement. However, it is probably a legal grey area if you create a new corporate entity for the sole purpose of being able to receive money
after you lost banking relationships with your main business.
Since my post yesterday an additional 30M of Tether have been released. I somehow doubt that institutional investors are buying all these Tether when they
could simply buy Bitcoin directly using a more reputable service like Gemini. Let´s assume that you are an institutional investor willing to buy Bitcoin. Why would
you wire 10M (or more) to Tether just in order to trade on Bitfinex when you could also skip this step and buy Bitcoin at Gemini or a comparable site
without the intermediary buy of Tethers?
I lend some USD there. I just simply used fiat to buy BTC, deposited them on Bitfinex, sold them and lent the money. What's the point Tether?