I should say that I am no expert on DMVPN but as far as I understand it by default internet traffic will be sent locally unless you tell it otherwise. The reason for this is the default route on the spoke which points to the ISP router so any traffic for which there is not a specific EIGRP route (in your case) received via the tunnel will be sent to the ISP ie. local internet.
The extra configuration is needed when you want all traffic, including internet, to go via the hub because then you need two default routes. The first points to the ISP router because without that the tunnel can't be built. But then you also need to send a default route from the hub down the tunnel so all traffic is sent via the tunnel interface and there is no local breakout.
You achieve this by using VRFs which gives you the ability to have two default routes, one in a VRF and one in the global routing table. If you do a search on DMVPN and VRFs you should find some examples.
As I say I am no expert on DMVPN but hopefully some of that has helped and hopefully others will add to this.
Jon
