Incorrect. They try to tell you that, or actually they tacitly admit they lack the authority to apply sales tax to those goods when they try to hide what it is behind the deceptive name "use tax". But it's exactly the same amount applied to the same goods and only when sales tax is not applied. It is a sales tax, but it's unConstitutional. If it weren't, they wouldn't even go to the effort of trying to fool you with the pretense.
I’m not sure what your definition of “unconstitutional” is. It clearly is not interpreted as unconstitutional, as most states have such a tax, and nobody has successfully challenged those laws. And given how people feel about paying taxes, if there was a chance that it could be unconstitutional, some lawyer would have done a class action suit long ago.
The biggest court case on this matter found that a state has no right to require a company that has no interests in a state to collect the sales tax for the state. The court did NOT say that the state couldn’t require those taxes to be paid, something that would have been germaine to the case if it had any merit.
I’m also not sure what “pretense” you are refering to. A “sales tax” is a tax collected when an item is sold. a “use tax” is a tax paid for the value of an item used. You pay “use tax” for items that are purchased for use by a resident within the state. It is specifically to capture the same tax as the sales tax; every example I’ve found is set to the same value, the taxes are described together in every state that I’ve checked. There is no deception about the relationship between the two taxes.