Taxing adventurers isn't a terrible idea, but they're doing it in a terrible way. Taxation is one method to prevent inflation. If too many adventurers became mega-wealthy, money wouldn't circulate, and it would affect the economy. The smart thing to do is to use tax exemptions to encourage them to circulate their excess wealth and direct it into areas the government deems beneficial. Things like donations to other adventurer schools, or dungeon materials research, or subsidizing public goods like healthcare or housing.
But if all you do is put an extremely high tax on adventurers based on their level instead of their actual achievements, it will make them either A) leave the country or B) overthrow the government.
We've had this plot about the government being greedy and incompetent since chapter 1. Before the dungeons, there was a power structure of wealthy elites and government leaders colluding to control society. Suddenly, their entire structure is replaced by "High levels are justice." The government is trying to maintain the old power structure by controlling the adventurers, but it's doomed. The material conditions for the old power structure no longer exist, and the new material conditions are level grinding.