This was true as far as the old Interstate & Defense Highway Act of 1956 was concerned. But the last 3 highway appropriation bills have made it legal for states to charge tolls on existing interstates. No one wanted to be first for fear of creating a political firestorm.
But Pennsylvania will be the first when it places I-80 under the Pennsylvania Turnpike Authority and erects 10 toll booths across the state. Others will soon follow.
IIRC, the PTA has already made its first payment to PennDOT, so I-80 should already be under the PTA’s authority.