Publicly traded companies can as well. There is no legal ruling that states a company must maximize EPS or face lawsuits. Apple could justify eating the margin to retain customers, build brand loyalty, or whatever other reason they can come up and the courts give a ton of leeway to business judgements in Delaware if they were sued over it.

Their fiduciary duty just means they have to act in the best interest of the company and its shareholders and not act for personal gain. Apple just has to spin eating the margin as being in the best interest of those parties.

The real reason they don't isn't any duty to shareholders, its any sort of drop would wipe billions off their market cap and directly affect exec compensation.

Amazon is right there, reinvesting profits.