Odoo was advised by Tax Cloud to do this.

Why?

In many states, applicable tax rates vary based upon the category of goods, the date of sales, the way goods are ordered in combination with other goods (certain combinations are tax free and others are not) and even the extended price of items being sold.

Because of this, many Sales Tax amounts cannot be converted into a single Percentage.  Doing so would confuse and mislead the Customer.

Examples:

• In the Commonwealth of Massachusetts, clothing items are tax-exempt below $175 (per item), however, every penny above $175 is taxed at .0625%. So, a $200 sweater would have a tax amount of $1.56 (which would divide out to an effective tax rate of .0078%).

• In the state of New York, shipping charges are taxable, unless everything being purchased is exempt. So if you sell $10,000 of t-shirts, and charge $1,000 for shipping, the entire $11,000 transaction is exempt. However, if you add even a single taxable item, say a $2 sticker, then your $11,002 transaction would be subjected to $88.93 of tax (shipping and the sticker attracting the NYC .08875% sales tax rate, however dividing that tax amount by the entire $11,002 transaction would suggest a tax rate of .008%).

Some states have partial rate reductions for items under or over specific dollar amounts, meaning the effective tax rate could be 6.5% for a specific dollar amount portion of the sales, and a reduced 1.23456% for the remaining portion.

TaxCloud is making some very complex computations (rounding rules, tax holidays, extended price considerations, regressive rates, product categories, etc.). A simple percentage rate wouldn't handle the complexity.

There are states where a single Tax Rate does apply, but since that isn't the case universally, Odoo agreed with the advice from TaxCloud and decided to hide the rate.