Emergency Radio essentially allows you to carry a police scanner with you at all times. Anyone interested in law enforcement or just looking for another “look what my iPhone can do” application should check it out. Source: Macworld.com
There are a number of websites that also do this same type of thing. Every now and again I get an officer come into my office appalled when he finds out that "anyone can listen" to their police radio traffic.
Yes, it is legal to listen to police radio traffic whether by a radio scanner, a website or even an iPhone application. FCC rules going back to the 1930's allow this. You should always assume someone, even a bad guy could be listening and keep sensitive information off the radio. Use your MDT, the telephone or an encrypted radio channel if it's really that sensitive.