The Tax option in Sales Order is available based on the Location of the Customer. If the location of the Customer is included in Non-Tax region, then option for Tax is not shown in the Sales Order.(Eg: US)
If the Location of the Customer is in Taxable region, then Tax will be included in the Total.(Eg:Canada)
The availability of Tax option depends upon the location of the Customer only and not depend on the location of the Parent Company.