Day rate on the US National Park system is US$10 - 15 per person and $20-30 per car. $6 for a Thai park fee is cheaper than a Coke at the concession stand in the US.
You forget that much of the revenue comes from VAT. It's the largest single category and makes up about 1./2 of total tax revenue.
I agree completely with you here. This is a legit gripe and one that should be addressed.