The solution is posted in the README file as follows:

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What about taxes?
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By default, your Retail customers (level 0) will be charged sales tax, and wholesale customers (level 1+) will not be charged any sales tax.

If you would like to charge taxes to EVERYONE, then do not include the following file in your installation (or, revert this file back to the original that comes with Zen Cart):
includes\classes\order.php