It's undefined behavior, and there's a bunch of people in here saying that server's have every right to violate DHCP leases, otherwise someone can take over a network by continually claiming all of a Class C by continually making DHCP requests.
That argument makes sense, but it goes very strongly against the grain of what I would think a DHCP lease meant, whcih is that it's a contract for a specific amount of time. If it is indeed a contract for a specific amount of time, the client has every right to claim what they're contractually obliged to. This was my initial assumption, and I believe it's Apple's rather valid assumption.
It's not a case of Apple not following the protocol. It's a case of the real world being more complex than the protocol.
Maybe a networking expert can correct me but it seems to me that those initial ARP requests before the DHCP request are not exactly in accordance with the protocol.