There are many similar cases, of course, and many are open to the "Why didn't you destroy the manuscript?" argument. "If you didn't want anyone to read your diaries, why didn't you burn them yourself, rather than leave that as a teasing injunction to your executors?" The implication is that the author really wanted them to be published, if only subconsciously, but that seems to me far from certain. Perhaps they wanted to be able to read them themselves, as long as they were still alive? That's a privilege people are usually accorded with respect to their own writing, without everyone else feeling they have carte blanche to peer over their shoulders. Or, if it's clear that the author really did want them destroyed, then some higher duty is invoked to justify the betrayal of their wishes - to Art, or to Posterity. But is this not bullshit of signal lucency?
In the case of O'Neill's play, he was quite clear that he did want the play preserved, and indeed published (in due course) - but not performed, so the "Why didn't you destroy it yourself?" argument doesn't apply. I don't know the details of how his wishes came to be ignored - not by future generations to whom he was nothing but a name, but by his widow, friends and colleagues, who might have been expected more than anyone to respect them. Perhaps they really didn't like him, and it was a form of revenge? That at least wouldn't be hypocritical. Perhaps it was to raise money for a life-saving operation? I could understand it, if so. But if it was just to satisfy the theatre world's curiosity, then phooey, say I.