In my Bitcoin Core Wallet I see peers identified as "BitcoinUnlimited" why?
BitcoinUnlimited is fork of Bitcoin project, maintained on github.

People using BitcoinUnlmited instead of Bitcoin-core are showing that as User Agent

BitcoinUnlimited offers two builds one for Bitcoin and other for Bitcoin Cash.

So what you are seeing is people using BitcoinUnlmited client for Bitcoin.

You don't have to BAN them, as they are not on BitcoinCash network but on Bitcoin Network.