Those racial incidents were on the football pitch, Cavani aside, so the FA getting involved is like the HR department or Ombudsman getting involved but not passing it on to the police, I can't think of any footballer prosecuted for on pitch racism.
Zouma's animal abuse was outside of football and has been referred to the police, the FA have no sway there. West Ham have fined him, more than most FA fines, and they didn't even have to do that. Plus, he still might be charged, despite what Dominic Raab says, the police do look back at crimes.
And why is Antonio wrong and Kirkland right? Do you think animal abuse in someone's own home is worse than racial abuse in front of millions of people?
Personally, I think Zouma should be prosecuted, but so should have Terry, Suarez, Big Ron, Emre Belözoğlu, Di Canio, Wayne Hennessy, Peter Beardsley, Bernardo Silva, Kiko Casilla and every racist fan.