Investors are not paying to hold Treasury bonds (yet), nominal yields are still positive (but that could change...). All that is happening is that inflation-protected securities are guaranteeing a return less than CPI inflation.
This is not that much of a mystery: there is no way to buy the CPI index (other than trading inflation derivatives). You can buy some goods, but you cannot lock in prices for services. As such, there is no guaranteed way to lock in the CPI inflation rate.
Investors try to make money trading bonds. As long as the nominal returns are greater than the nominal cost of holding positions - which is approximately equal to the policy rate - buying bonds is profitable. Current U.S. Treasury pricing is consistent with the belief that the Fed will cut rates.
The interesting question is why investors would hold bonds with negative nominal yields. The reasoning is more complex, but as we see in Japan and the euro area, they can be forced to do so - since holding large amounts of currency notes is not an option for institutional investors for security reasons.