I live in Florida. In Florida you get to renew your vehicle registration on your birthday. My birthday just happened, so I sent the state of Florida $57.00 so that I can drive for another year. Fifty seven dollars is five percent of my monthly income.
Now, I don't mind paying five percent of my monthly income once a year for the privilege of driving. It's worth it to me. But I don't think it's fair that I have to pay 5% of my monthly income for the privilege when people who drive WAY more than I do (I drive something like 400 miles a month) pay 1% or .1% or .0001% for the same privilege.
When rich people complain about progressive taxes and the state responds by instigating "fees", less-than-rich people bear the brunt. And poor people get totally f'd.
8:49 PM PT: I just edited the title to be more specific. My issue is with flat fees, not fees that have some kinda "sliding scale" component.