$100M in deposits is $5M/yr in returns on float, just as your bank and insurance companies do with your deposits and premiums. It's technically not risk-free (FDIC notwithstanding) but that doesn't stop anyone.
Even treasuries are not risk free, nor are they unbounded. Tether is trying to play in the big league, I'm not certain what the rules are, but I'm pretty sure making 1%-2% annually on the float is a pretty good deal, and that might even be a conservative estimate.
In addition you have to keep in mind that it is also about absolute numbers. No one invested these billions with them, no one is charging them some kind of relative rent. Every dollar is profit. If you could make 0.001% off storing 5 billion, that would still be an attractive business model.
“[...] each tetherUSD in circulation represents one US dollar held in our reserves (i.e. a onetoone ratio) which means the system is fully reserved when the sum of all tethers in existence (at any point in time) is exactly equal to the balance of USD held in our reserve.”
Exactly. And even if the US government were to default on loans Theter would be the least of your concerns since the USD and consequently the global economy would be in a pretty bad shape.
You can sell them before maturity, but their value will depend on current interest rates, of course. If rates are up, you will have to discount the price.
This might not be exactly answering your question, but you can buy 4 week treasury bills. Even if you can't sell bonds, having money tied up for 4 weeks might not be a big problem.
It's the gamble that banks make. A bank holds your money and will give it to you at any time. To fund this, they make an assumption about how many of their customers will actually want their money at any one time, and they invest the rest.
If all their customers decide to withdraw their money, this is a "run on the bank" and generally dooms the bank.
Presumably Tether planned to do the same: a percentage of the money they held would be invested, based on the assumption that not everyone who held Tether would want USD at any one point in time.
Of course, this stretches the meaning of "reserve", and utterly fails if they mismanaged the investment bit and lost the money they invested