Since Java 1.5, there has been an interface called ProxySelector:

http://docs.oracle.com/javase/7/docs/api/java/net/ProxySelector.html

The default implementation of this provided by the JDK uses the Java system properties to decide whether to use a proxy server or not. If AsyncHttpClient switched to this when useProxyProperties was set, it would mean less code to maintain, and it would also mean that things that AsyncHttpClient doesn't currently support, such as wildcard matching in http.nonProxyHosts, would be supported.