The IRS has to have some basis in fact to say you have more income than what you earned. After all, if you contested hard enough by taking it to court, the IRS will have to have their ducks in a row, and normally, they aren't in the business of making themselves look foolish in court.

The burden of proof isn't as drastic as you make it out to be. If the IRS thinks there's unreported income, they'll send you a notice of proposed changes, and ask you to provide any additional information. If they don't hear anything, they'll issue a notice of deficiency, which will give you 90 days to file a protest, which just means that somebody within the IRS will review whether the notice is correct or not.

The whole "burden of proof" issue usually is invoked for people who do nothing to respond to these notices and think that they don't have to do so in order to win a fight with the IRS.

(For disclosure purposes, I'm a low-level grunt for my state's income tax authority, and our procedures pretty much mimick those of the IRS.)