Yes Juniper is different. In older EX products, non-ELS, you needed to enable DAI first in order to get IP-SG to function. In new EX, like EX4300 with ELS (different chip set) when you enable DHCP related security features, DAI gets automatically turned on (does not require any specific confih knob to be turned on) when the security feature is set.
So when you enable IP-SG on Ex4300, DAI is also automatically enabled.