[ghaff]

The presumption is that you, as (say) a local retailer, collected sales tax on what you sold because you have a legal requirement to do so. Doesn't matter if you split it out or not, you effectively collected sales tax one way or the other because state law says you had to. If, in your mind, you didn't collect any sales tax, too bad because you had to. You don't have the option to tell the customer "Hey, we're not going to collect sales tax. Please send it in yourself."

If you weren't required to collect sales tax, it is actually the customer who is on the hook to remit it. Of course, there's often no way to enforce this.

>Edit: I think you are confusing it with income tax. What you are saying happens for salaried income in USA.

Sort of but not really. Employers collect an estimated income tax liability based on your salary and what you've put on your W-4. The individual is still responsible for paying the right taxes and may be subject to penalties and having to make future quarterly tax payments if those withholdings are too low.

ADDED: The difference is the the government (in general) knows your taxable income and it's up to you to make the appropriate payments. In the case of sales tax/use tax, the government (thankfully) doesn't have the detailed and personal information to determine liability at the individual level so they have sellers do the up-front collection as much as possible.