The theta vacua$^1$ of a Yang-Mills quantum theory are given by $$|\theta\rangle=\sum\limits_{n=-\infty}^{\infty}e^{in\theta}|n\rangle.$$ In Srednicki's Quantum Field Theory, he claims that the energy of a theta vacuum is proportional to $-\cos\theta$ so that the theta vacuum labelled by $\theta=0$ has the minimum energy. For other values of $\theta\neq 0$, the corresponding theta vacua $\{|\theta\rangle\}$ will have higher energies. Why are then the states $\{|\theta\rangle\}$ with $\theta\neq 0$ are referred to as vacua?
$^1$ As far as I understand, due to the possibility of tunnelling mediated by instantons the states $|n\rangle$ are not the true ground states of the theory. Gauge field configurations labelled by $n$ are minimum energy configurations only in the classical theory.