That's just an indication of their safety. Short-term U.S. Treasuries are being used as a safe bank account by a number of people with money to park who don't trust regular banks, and don't want to expose themselves to the interest-rate risk that long-term bonds bring. This raises demand and pushes down interest rates, sometimes even into the negative range.
If you're an American with, say, $75k to park, there's no reason to put it in a treasury at negative interest, of course: you can just put it in an FDIC-insured bank account. But if you're a Cypriot with $50m to park, buying treasuries looks attractive relative to Eurobonds or Cypriot banks, and continues to look attractive even if prices rise to the point where the interest rate is moderately negative.
But in neither case should you buy a 30-year bond for short-term cash parking, unless you are either hedged against the interest-rate risk, or willing to expose yourself to a bet on the direction interest rates will move. If they move the wrong way, your $100k might be worth $90k next year, which will completely wipe out your 3.5% interest and more.
Those were not bonds, but rather Treasury Inflation Protected Securities (TIPS) which are tied to CPI.
The fixed payment on five-year TIPS, known as the real yield, has been pushed below zero because the rise in the CPI is greater than the yield on regular five-year U.S. notes
