The problem is that it would create two very big problems. First, in the world after the 1965-Watts Riots, many people are going to equate police violence to "legal lynching". This would be particularly underlined in the states below the Mason-Dixon line wherein police officials actually participated in African-American Lynchings throughout the first half of the 20th Century (see: Incognegro by Mat Johnson). To make matters worse, for many LGBT people, the vigilante-style of justice is synonmous with gay-bashing, especially after the 1969 Stonewall Riots. For many Latino/Chicanos, the police violence is an unkind reminder of things like the Zoot Suit Riots of 1942,the 1954 Operation Wetback, or the 1994 Minuteman Militias. In any event, "Dirty Harry" justice usually means a reputation of corrupt racist, misogynistic, and sexist "loose cannons" with no respect for the law....
The second problem is that this easily justifies the escalation in violence that comes from these tactics. Fors instance, the 1965 rise of the Black Panther Party in Oakland, California was started as a result of the police brutality of local officers. The Symbionese Liberation Army was formed in 1973, because of perceptions of police brutality in San Francisco and Los Angeles, California. The Weathermen (a.k.a. the Weather Underground) was built in 1968, on the basis that the country was being "run by jack-booted stormtroopers". A "Dirty Harry" universe would cause a massive escalation, wherein the Bloods and the Crips would be romanticized even further than in OTL, because of the violence brought by the police....