Yep, they are under the illusion that a policeman will always be where the crimes are as they are being committed.
Yes, some do suffer from that delusion. They're insane and stupid.
Others know perfectly well that the police cannot protect the individual citizen; for them that's part of the plan. The plan is to centralize all power in the Government Almighty (which these people control). Any individual power, such as the power of self defense, which they do not control must be suppressed. This desire to suppress the individual is at the heart of all leftism. Such people as these are evil.