These internships are so highly prized by young people planning a political career congressmen could charge a premium for the job and people would pay. I’m sure that happens under the table. Of course these jobs are limited to the independently wealthy or people with a wealthy sponsor.
This tends to be the way it is. Most internships in DC are not only for no compensation, but require the person to pay for all travel with the campaign on their own dime, including flying back and fourth from the district to DC, and for maintaining a residence in DC. They sometimes even charge for the opportunity under the table.