In the US anyway, that would be illegal. Dollars must be accepted for all debts, public and private.

There's no debt if they refuse to sell you anything.

If the stores are not forced to use Bitcoin and are still allowed to accept other forms of payment but simply choose not to, then yeah, it's voluntary.

Go find someone who's willing to take whatever you're trying to use instead, why should other people have to take your form of payment against their will?