I don't think that there is a notice in the airport informing visitors about this?
What kind of authority will allow this kind of taxi driver to stand in the lobby of an international airport (at least this is how it called) to rid off foreign visitors?
They don't. However, the lobby is a public place. There are periodic raids/inspections by the police but there's not that much they can do. However, there are certainly a lot fewer of the mafia drivers now than there were ten years ago.
And it's not as if Warsaw is the only city in the world with this problem. A Polish man I know was last year charged £175 for a mini-cab (not a black taxi) from Heathrow to Chiswick!
It only depends on your standard. In
Drinking water in Warsaw is not harmful for your health at all.