  Asymmetric destruction of the one-third magnetization plateau upon heating is
found in the spin-1/2 kagome Heisenberg antiferromagnets using the typical pure
quantum state approach. The asymmetry originates from larger density of states
of low-lying excited sates of $N_{\rm s}$ spin systems with magnetization
$(1/3-2/N_{\rm s})$ than that of low-lying sates with magnetization
$(1/3+2/N_{\rm s})$. The enhanced specific heat and entropy that reflect the
larger density of states in the lower-field side of the plateau are detectable
in candidate materials of the kagome antiferromagnets. We discuss that the
asymmetry originates from the unprecedented preservation of the ice rule around
the plateau.
