It's plainly fucking unamerican.I'm not so sure about that. Grand juries traditionally operated in secret, and subpoenaing information on someone without tipping them doesn't necessarily violate the constitution. Now if someone is charged with a criminal offense, they have a constitutional right to confront witnesses under the 6th amendment. but that's not the same as a requirement to be notified of being the subject of an investigation where no charges have been brought.
Note that the grand jury system itself is is explicitly provided for in the Constitution - one of the less well-known aspects of the 5th amendment, which requires that prosecutors make a case before a grand jury before attempting to bring suspects to trial in serious cases. I'm not an expert on the history of grand juries, but I believe this summary from US v. Johnson (319 U.S. 503 (1943)) suts things up well:
Were the ruling of the court below allowed to stand, the mere challenge, in effect, of the regularity of a grand jury's proceedings would cast upon the government the affirmative duty of proving such regularity. Nothing could be more destructive of the workings of our grand jury system or more hostile to its historic status. That institution, unlike the situation in many states, is part of the federal constitutional system. To allow the intrusion, implied by the lower court's attitude, into the indispensable secrecy of grand jury proceedings — as important for the protection of the innocent as for the pursuit of the guilty — would subvert the functions of federal grand juries by all sorts of devices which some states have seen fit to permit in their local procedure, such as ready resort to inspection of grand jury minutes. The district court was quite within its right in striking the preliminary motions which challenged the legality of the grand jury that returned the indictment. To construe these pleadings as the court below did would be to resuscitate seventeenth century notions of interpreting pleadings and to do so in an aggravated form by applying them to the administration of the criminal law in the twentieth century. Protections of substance which now safeguard the rights of the accused do not require the invention of such new refinements of criminal pleading.