One aspect to consider is that at American Express the specific transaction which crosses the bonus threshold is special. For example, if you need to spend $3,000 to earn a bonus, and you buy $500 airline ticket, then pay $2,500 taxes, the tax transaction is tagged, and may be examined. If you made those purchases in the reverse order the airfare would be tagged. If you receive ANY refunds or credits before earning the bonus it seems to trigger an examination of the spending, initially automated, sometimes manual. It may be that some auditors believe that tax payments are Cash Equivalents, but nonetheless many people use tax payments to earn bonuses without any controversy.
this has been my experience also. The two times I had problems was with tax payments that put me over the threshold - one was real estate taxes and the other federal. The federal one I managed to appeal though it took 3 months, but that was about 4 years ago so things might have changed as at that time tax payments were not explicitly excluded.
On three other bonuses where the tax payment was earlier in the spend cycle I had no problems.