The basis of this probably comes from page 3 of this:
PHMSA
These guys have also been pushing the fee hikes in other areas since 2008; like raising the per person registration fee from $975 a year to $2575.
These costs are passed on to us the consumer via the shipping company as the HazMat fee.
Edit to add: They are a regulatory agency. They do not necessarily need a new law or congressional approval to hike their fees.