No where in the code does it say breakers are only rated for 80%.
Actually it does say that for continuous loads in a round about way.
210.20 (A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.
Exception: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
Also if you look in the UL General directory you will find that breakers unless otherwise marked, are only listed for 100% operation of loads lasting less than 3 hours.
A typical 20 amp breaker can be loaded to 20 amps for a period up to 3 hours or 16 amps forever.