You have to realize that the US can only prosecute violations of American law. I'm sure folks in Russia are doing all kinds of things that are illegal in Russia, but we can't do anything about it.
Kickbacks of US foreign aid to the Ukraine, to Hunter Biden, is covered by US law.
{...
§ 1.162-18 Illegal bribes and kickbacks.
(a) Illegal payments to government officials or employees -
(1) In general. No deduction shall be
allowed under section 162(a) for any
amount paid or incurred, directly or indirectly, to an official or
employee of any government, or of any
agency or other instrumentality of any government, if -
(i) In the case of a
payment made to an official or
employee of a government other than a
foreign government described in subparagraph (3)
(ii) or
(iii) of this paragraph, the
payment constitutes an illegal bribe or kickback, or
(ii) In the case of a
payment made to an official or
employee of a
foreign government described in subparagraph (3)
(ii) or
(iii) of this paragraph, the making of the
payment would be unlawful under the laws of the
United States (if such laws were applicable to the
payment and to the official or
employee at the time the expenses were paid or incurred).
No deduction shall be
allowed for an accrued expense if the eventual
payment thereof would fall within the prohibition of this section. The place where the expenses are paid or incurred is immaterial. For
purposes of subdivision
(ii) of this subparagraph, lawfulness, or unlawfulness of the
payment under the laws of the
foreign country is immaterial.
...}
www.law.cornell.edu