MIT was concerned that someone (they didn't know who it was at the time--in fact IIRC they didn't find out until the police told them after they caught Swartz) was using their network in a suspicious way. I don't know that anyone was "angered".
> You can set a surprising amount of government force in action with a concerned phone call, as it turns out.
I don't think the people at MIT who called in the police (and they were, IIRC, fairly low level people) had any idea what a shitstorm they were going to unleash. Nor do I think the (higher-level, but still IIRC not at the level of the top MIT administration) people at MIT who ended up interacting with the Federal prosecutor thought through what was actually going on as they should have, or had the issue considered by the MIT administration as a whole as it should have been.