Just kidding. Got some feedback from people knowledgable about park rangers and why they carry firearms.
From my friend, the former national park ranger:
Because the parks, like the rest of the country, are full of criminals some of whom want to hurt people, transport/manufacture drugs, steal archeological artifacts, smuggle aliens etc. ad naseum.
From someone familiar with law enforcement in Delaware:
Park Rangers go through the State Police Academy.
Park Rangers enforce and prosecute all state laws.
Park Rangers are real Police.
I guess I associate park rangers too closely with Smokey the Bear to really think of them as law enforcement officials, but I can see why they are now.
UPDATE: My former park ranger friend is embarassed by what she wrote above due to spelling and "homonym" issues. Not that I noticed them myself, so who's the real "moron" to use her term, the person who wrote it quickly while running late to work or the person who didn't even notice himself? Rest assured, she's very intelligent, despite the impression you may or may not have received above.