Foreign Registration Is A Matter Of Fairness
If you are from another state and want to do business in Idaho don't let yourself feel victimized by the requirement to register before doing business in the state. This same requirement exists in every other state and is in place for many reasons. One of the most important reasons is a simple matter of fairness.
For example, a plumber that lives in Idaho is required to pay state income tax in Idaho. Those taxes paid by Idaho residents and companies go toward Idaho roads and schools and so forth. So if you are coming from another state to work in Idaho, it is only fair that you pay tax on the income you earned on an Idaho job to the State of Idaho. Otherwise, you would have an unfair competitive advantage over Idaho residents that pay their taxes as well as the other out-of-state entities that register and pay taxes on the work they do in Idaho.
Consider a case where an out-of-state plumber from Washington doesn't register in Idaho as a "foreign" filing entity and doesn't pay income tax for his or her work done in Idaho. In addition to being illegal, this results in a distinct competitive advantage over a plumber from Idaho when you consider the two different profit margins between the Idaho plumber who pays taxes and the hypothetical, unregistered Washington plumber who intentionally does not pay tax on work done in Idaho.
The practical effect of this is that the unregistered plumber from Washington can more easily under-bid an Idaho plumber, or, for that matter, a different Washington plumber that pays taxes on the work she does in Idaho. It simply would not be fair to tradesmen and women that operate legally in Idaho for someone to under bid them on a job because they don't pay their taxes.
Our hypothetical unregistered, tax dodging plumber from Washington can far more easily under-bid an Idaho plumber that pays his taxes and still have the same take-home pay for the job than the Idaho plumber could have had if his higher bid had been accepted. That is simply not fair. It also means Idaho residents get stuck with the tab for all the un-registered/un-taxed revenue that goes out of state instead of being paid by a legally operating "foreign" plumber who is on equal ground with an Idaho plumber.