Use tax applies on things that would be normally taxed under the sales tax rules but the sale didn't take place in Idaho. If a company has a physical presence in the state, they will charge you sales tax on your purchase over the net anyway, as they are required to by current law.
Most states have a use tax to go along with the sales tax to cover sales to you not made in the state.
As an aside, for all of those saying consumption tax is the most fair, then to consume and not pay the tax, it is a bit hypocritical. Don't get me wrong, I'm not for more taxes. It is just an observation.
That part of their brains is missing.