You focus on isolated incidents where there is either negligence, poor management, or corruption.
I am making a general assertion. Show me your supposed exception to the rule, and I'll show you a special interest group behind it.
It does exist and should be scrutinized, fixed, or punished... but there are also many examples of good people doing good work... most national parks I've visited display what I'm talking about.
US national parks? Hilarious, because that's where all the real lobbying money is at. It's only protected when its relative worth is below a certain level of profitability. That is if you really want to call them protected, since they even allowed limited economic activity within many of these parks now. Sometimes they will outright eliminate protected land.
Did you know that the majority of national parks produce a direct revenue stream for the government? Did you honestly believe that the highest level politicians and bureaucrats have a hard on for the NPS? No, don't be so naive.
That government shield of yours is air thin. Even the most environmentally conscious countries, like Costa Rica, are rampant with corporate exploitation of the environment. Whenever the tourism industry in these countries takes a hit, the government will pass new legislation allowing more abuse of the land and wildlife.
Please, do me a favor. Explain how an institution which serves only money, can ever be expected to have the best interests of the environment in mind?