This is what regulation is for. No regulation? Then people are stealing. History suggests it's that simple.

How would they prove it? Twitter a picture of the bank balance?

At the very least, inform the public about which banking institutions they are currently using ever since Wells Fargo dropped out.

Contracting a reputable firm to conduct an in-depth audit and publicly releasing the results would also be a useful step.

And not refuse to name banks without an NDA signed and in place...

Did I misread the article, or weren't the banks they refused to name the banks involved in shutting them out of the US?

As far as I believe Wells Fargo is the bank that severed ties:

"After Wells Fargo severed ties, Tether won’t name its banks"

"On Dec. 2, Bitfinex released a quarterly report announcing it would no longer serve U.S. customers because it’s too expensive to do business with them. This followed Wells Fargo & Co.’s decision earlier in the year to end its role as a correspondent bank through which customers in the U.S. could send money to Bitfinex and Tether’s banks in Taiwan. Bitfinex and Tether filed suit against Wells Fargo, but later withdrew the case."

"However, Ronn Torossian, a spokesman for Bitfinex and Tether, refused to identify their current banks unless a reporter signed a non-disclosure agreement, an offer that wasn’t accepted."