in many areas, radio amateur volunteers support public safety through official agency programs such as RACES and communications auxiliary units, and other local disaster volunteers (ARES, etc). Therefore, it is logical that hams many have scanners, or amateur radio equipment capable of receiving public safety frequencies (most VHF/UHF ham radios made in the 20 years do this), so such laws have exemptions for amateur radio operators.
Many SKYWARN spotters are also licensed amateur radio operators, and use scanners to monitor public safety during storm seasons and relay reports via amateur radio to the National Weather Service when they hear confirmed touchdowns of tornadoes and other storm damage reports.