Chris Hamm

The key language seems to be in the GAO Redbook, Chapter 4, Section 5: Entertainment—Recreation—Morale

and Welfare:

The concept to be explored in this section is the rule that appropriated
funds may not be used for entertainment except when specifically
authorized by statute and also authorized or approved by proper
administrative officers. E.g., 69 Comp. Gen. 197 (1990); 43 Comp. Gen. 305

(1963). The basis for the rule is that entertainment is essentially a personal
expense even where it occurs in some business-related context. Except
where specifically appropriated for, entertainment cannot normally be said
to be necessary to carry out the purposes of an appropriation.
The reader will readily note the sharp distinction between government
practice and corporate practice in this regard. “Entertainment” as a
business-related expense is an established practice in the corporate sector.
No one questions that it can be equally business-related for a government
agency. The difference—and the policy underlying the rule for the