Are altcoins private companies?
I think you are confusing the monetary function of "coins based on blockchain technology" with the technology itself.  Blockchain technology, and thus any "alt-coin" using that technology, is used to prove the chain of ownership of a some item.  In the case of Bitcoin, that item is money, however it might just as easily be used to prove ownership of oranges.  

Blockchain technology can be used by anyone to create a proof-of-ownership chain.  Therefore, a private company might choose to use it for whatever purpose they desire; that may make it closed, private, non-neutral, and non-censorship resistant.  That doesn't make it wrong, per se.

In the case of "alt coins", always perform due-diligence before making an investment.  Unless a company is designated as "not for profit", practically-speaking the only reason for it to exist is to make money for its owners.