Tether sold tokens on the promise that 1 USDT would be backed by U.S. dollars. They didn't keep one U.S. dollar for each USDT sold. If they did (or concealed) that willfully, it's fraud.

> Why is it not possible to back cryptos to a stabilized "1:1" "medium/currency" (which itself is another crypto it seems) and finally the USD dollar ?

U.S. dollars can be held as hard cash or in bank accounts. Holding $4 billion physically is bonkers. Banks holding U.S. dollars, meanwhile, have to follow U.S. anti-money laundering law. That requires, among other things, knowing who beneficially owns their deposits.

If an LLC opens a bank account, the bank will ask about its owners. With Tether, the beneficial owners are USDT holders. Since Tether can't identify them, they can't satisfy any bank's compliance questions.

So either (a) Tether's banks are violating U.S. AML law, (b) Tether is lying or (c) Tether is silently restricting and keeping records on who owns every USDT. Given we have contrevidence for (c), one can conclude–based solely on Tether's claims–that there's fraud afoot.

Disclaimer: I am not a lawyer. This is neither legal nor trading advice.