The net bottom line here, all descriptive terms aside, is the same one that even today most children understand without any formal instruction: in a 'fair' game, or -- generalising -- a ''fair'' society, you do NOT change the rules after the game begins.
Fair enough. One condition any employer will demand of a prospective employee--before the job is accepted, rendering it acceptable according to your philosophy--is that the employee agree to accept any later changes made by the employer to the terms of employment or face termination. The employee knows going in that the terms may be unilaterally changed, and if he doesn't like this condition, he is free not to accept the job, so there is nothing unfair about the employer subsequently changing those terms.
An a posteriori condition, made at will and at whim by the company subsequent to employment, is simply unacceptable from any standpoint you choose to name, including that of private property.