I offered you a plain donut, you accepted a plain donut, that's the contract. Offer and acceptance. And that would probably be the last free donuts the office got.
Now in plain fact YOU didn't offer anyone a "free donut": the corporation did. This is a critical distinction.
Corporations exist solely by virtue of Nanny State interference in the operations of the Free Market.
This gives corporations--which offer internships--a vastly privileged position in the negotiations they undertake with potential employees, interns, etc.
Again: corporations are a privileged form of social organization by statute (the reforms to the Companies Act in Great Britain in the 1850's, and similar acts passed by parliaments and congresses around the world.) I own a corporation, and when I incorporated I did not engage in free an uncoerced trade with my fellow humans: I filed forms with the government that upon approval gave me as a corporate owner certain legal, state-defined and state-protected privileges that my employees do not have the benefit of.
Advocates of Corporatism like yourself tend to forget this little detail: you as the owner or agent of a corporation have the backing of the massive, coercive power of the State. Your employees do not.
So quit pretending you live in some mythical Free Market where the Nanny State hasn't tilted the scales massively in your favour. Show a little humanity and humility and decency, and remember that what the State giveth the People can damned well take away.