This feature is disabled to provide as much security as possible from the default configuration.

Because proxy ARP allows hosts from different LAN segments to look like they are on the same segment, proxy ARP is only safe when used between trusted LAN segments. Attackers can take advantage of the trusting nature of proxy ARP by spoofing a trusted host and then intercepting packets. Due to this security weakness, most vendors (including Cisco) recommend that you always disable proxy ARP on router interfaces that do not require it.