The amount of money that a person has in a bank account is irrelevant to determining tax liability. In order to assess income taxes, the IRS needs only an accounting of the interest, dividends or capital gains that a person has earned as income. Therefore, the filing requirements of FBAR, FATCA and 8938 are unrelated to the need to assess a person’s tax liability. The information, however, could possibly be useful to detect a crime of money laundering, terrorism, counterfeiting or criminal tax evasion, the very crimes that the Bank Secrecy Act is meant to curtail–that is if the IRS could figure out how to get terrorists, money launderers, counterfeiters and criminal tax evaders to fill them out. Since they haven’t figured out that one, the FBAR law is useless for any legitimate purpose.
The Fourth Amendment protects the privacy of individuals to the point that if a government officer wants to search a persons papers or property, for the sake of determining if a crime has been committed, he must have a search warrant obtained on the basis of probable cause. It is a prohibition of general warrants. It seems to me that the FBAR, FATCA, and form 8938 circumvent the necessity of a search warrant on probable cause and are thus the same, in essence, as a general warrant.
Here is an 19th century explanation of the 4th amendment rule regarding search warrants:
Flanders, Henry. An exposition of the Constitution of the United States : designed as a manual of instruction. Philadelphia, 1860, p. 259-260
433. The fourth amendment provides that the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, should not be violated ; and no warrants shall issue but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
434. This refers only to warrants and process issued under the authority of the United States, and was doubtless intended to guard against the practice which formerly prevailed in England, in one class of cases, of issuing warrants in a general form, which authorized the officer to search houses and arrest persons, without naming any persons or places, in particular. This was a sort of roving commission to the officer to search any place and arrest any person whom he might suspect of being the accused party. The King’s Bench, in the time of Chief Justice, Pratt, pronounced general warrants to be totally illegal, and Parliament subsequently passed resolutions against them; resolutions by which the personal liberty of the subject was confirmed, and the lawful secrets of business and friendship were rendered inviolable.
435. Under the Constitution of the United States a warrant cannot issue but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the person or things to be seized. The warrant, and the complaint under oath upon which it is founded, must not only state the name of the party, but also the time, and place, and nature of the offence, with reasonable certainty.