To clarify: TaxCloud is NOT responding with the wrong rate. Rather, TaxCloud is responding with the correct tax due under Colorado law.
Period, end of debate.
However, what you are complaining about is the accommodation Colorado allows for local compliance only (ignoring counties outside of a retailer's home county/city). This accommodation is valid under current CO law, but this accommodation is based upon a clearly out-dated belief that keeping track of many jurisdictions would be unduly burdensome (in the words of the 1967 Supreme Court Bella's Hess opinion). jUst because the accommodation is available, it does not change the legal fact that the full sales tax is due - the accommodation merely shifts the burden of compliance from the retailer to the consumer. Colorado even recently enacted modifications to this law to modernize and simplify compliance, which is why TaxCloud now works in Colorado as well (though we have not been certified yet).
With TaxCloud, our situation is absolutely transparent. We are paid by the states to make sales tax compliance as easy as possible for retailers - that is why our service is FREE. Our mission is that no retailer should ever have to pay to comply with sales tax laws - the states should. Given that our ultimate employers are the states themselves, retailers benefit with full indemnification, handling of notices and audit inquiries, and even automated remittance of sales tax proceeds in states that have certified us (more than half of the sales tax states as of this moment - but admittedly not CO yet).
So, if you want to easily comply with sales tax laws, we are your best option. However, if you are intent on exploiting the "undue burden" loophole from from 1967 by refusing to collect sales tax legally due, than TaxCloud will not be a good option.