Nepal is a low income and high corruption country, where the government and formal business structures are unstable enough that 'tipping' becomes common even for government investigators...
It's basically a way for everyone to get more tourists dollars, which is one of Nepals primary exports.
Scamming bored rich people is one of the more victimless crimes.
Why don't they just charge [more] for a mountain license? A few thousand per hiker would probably be tolerated by people who view the hike as a lifetime achievement kind of thing.