What would a verbal warning have been useful for? He already knew his behavior was unwanted, and that's what verbal warnings are for. He wasn't stupid, so the verbal warnings wouldn't have been the first time that he learned that there were actual laws against unauthorized network access.

Besides, what's there to warn him about, he's 100% right in what he was doing, wasn't he? To hear it from the hacktivists, he did nothing wrong (and so there's nothing to warn him about). If he did do something to warrant a warning then he was already past the point of needing one.

First, by talking to him, MIT would have revealed that they knew who he was. That alone might have dissuaded him, as he was clearly acting under the assumption that he had successfully hidden his identity (or otherwise he would not have bothered to hide his face from the camera).

Second, they might have learned his motives and either found another way for him to achieve his objective (perhaps through some sort of compromise) or persuaded him to stop.

Third, they could have made clear to him what the penalties the government was ready to use against him in this case were, and that they had enough evidence to take him to court if he persisted.

Any of these might have been enough to lead to a less tragic outcome than what wound up happening when they immediately threw the book at him.

That's definitely not true. Everybody knows that speeding is a crime. But cops give warnings all the time if you didn't harm anybody and they think you're a reasonable person who will listen to them.