The idea that banks are private companies in any country is not entirely true. Everyone knows that if a reasonably big bank fails, governments will intervene to protect people's money.

Banks are technically private but won't be valued as much without government protection. So, to some extent, it's not wrong for the government to tax them more than other private companies.

However, a negative consequence of this tax it reduces the bank's incentive to lend money to small and risky companies/startups. This is equivalent to increasing capital gains to 40% - why invest in risky assets if I know I will ever only receive a small percentage of my profits (if any)?