Yes, I knew that the transaction value (and source, sink) was known after processing it and putting it in the blockchain, just not that it was known by the miner processing the transaction before putting it on the blockchain - I assumed when "mining" a transaction that somehow the details would be hidden.

Them being visible seems to raise interesting attacks, such as an individual being doxed and then unable to have their transaction verified/accepted.

A sibling comment suggests Monero have addressed this.

Edit: If 51% agree not to process any transactions for accounts holding >$1M worth of Btc can they steal financial value? Like a stock buyback but you pay zero to the holder and get the stock anyway.