Eh, rules I suppose - I know that's a rule in American sports, wasn't aware that was forbidden in the Prem.
However, if he and his agent believe Everton are lowballing him, they can quite rightly decide to not sign the contract and take their chances elsewhere. My main issue is with it being a public spectacle, because if he signs now I think it will have impacted his relationship with fans in a way that may affect his production, being such a confidence player.
Which is why I believe it's either a stupid move, or Koeman actively pushing him out.