It's pure politicking: Apparently, Senator Grassley thinks that the IRS can and should effectively crack down on prostitution.
I completely agree with this:
"If governments actually cared about the victims of sex trafficking, the logical thing to do would be to legalize and regulate prostitution. A legal, transparent system would make it much easier to ensure both the age and ability to consent of prostitutes. Abuse would go down, disease incidences would go down, and child trafficking would go down. Prostitution will always be with us, so why not ensure that any acts of prostitution occur solely between consenting adults?"
Libertarianism at its most logical.
(trust me—I don't get many chances to say that)