That one enters into implicit contracts is undoubtedly correct, however I would question the duration of that contract.
Firstly, the benefit relationship of all contracts, implicit or otherwise, is asymmetric: the benefits accrued to the employer will be greater than those accrued to the employees: to do otherwise would erode profits, and put the company out of business. Some companies have 'enlightened' opinions about how to extract good work from their employees, but it is still done with the goal of being more profitable. Arguing from the poor practices of other companies is not constructive.
With respect to the particular situation, once the employee had been placed on notice, it seems to me that the implicit contract was broken, unless the employer was just as willing as they were beforehand to offer the 'implicit benefits' to the employee, which seems unlikely. Indeed, the situation really seems to be that the employer is asking for all the benefits on their side of the implicit contract, without offering anything in return.
Finally, for me the implicit contract is not really between the employee and the employer, but between the employee and their coworkers. Whenever I have gone above and beyond, I haven't been doing it for the company, but for the people I work with. However, my experiences are limited to medium sized companies, so things might be different in smaller environments.