The Department of Justice publishes a guide for its lawyers called the United States’ Attorney Manual (USAM).

Section 6.4-000 comprises the Criminal Tax Procedure portion of the USAM.

Here’s the preamble of 6-4.010:

The Government helps to preserve the integrity of this Nation’s self-assessment tax system through vigorous and uniform criminal enforcement of the internal revenue laws.   Criminal prosecutions punish tax law violators and deter other persons who would violate those laws.   To achieve maximum deterrence, the Government must pursue broad, balanced, and uniform criminal tax enforcement. Uniformity in tax cases is necessary because tax enforcement potentially affects more individuals than any other area of criminal enforcement.   Broad and balanced enforcement is essential to effectively deter persons of varying economic and vocational status, violators in different geographic areas, and different types of tax law violations.   To achieve uniform, broad, and balanced criminal tax enforcement, the Attorney General has authorized the Tax Division to oversee all federal criminal tax enforcement and to authorize or decline investigations and prosecutions in tax matters.  

Read it again.

Here  you have the general marching orders given by the U.S. government to those charged with prosecuting violations of America’s federal tax laws.

The two most important words here are “deter” and “uniformity.” U.S. Attorneys must  discharge their responsibilities with an eye toward deterring others from violating the law, but it must do so in a manner that is “uniform and balanced.”

In general, uniform and balanced means that the United States must treat similarly situated tax violators similarly.