If you committed that crime, the police would probably try to find you and stop you, not just give up on anonymous tips :) As you said, they're obligated to respond, as they have a monopoly on violence.

How many people do you know who would get a threatening email about imminent violence and play it off because it's likely a fake? Perhaps some the most zen among us! The rest of us would almost definitely want to (hire mercenaries to) take reasonable steps to stop it. A few hours of police time in exchange for preventing a potential terrorist event seems like a fair trade, anyway...