What's happening? Well, normally with DHCP, the client is given a default route as part of the information exchange. That eth0 default route takes over. It's just that simple... unless you give the networking system guidance on what to do in the form of the "DEFROUTE" option. The option has been around for a long time. The current Red Hat Enterprise Linux 6 Deployment Guide has DEFROUTE option hidden in the 8.2.4. Dialup Interfaces section. Here is a chunk of the information from the guide:
answeris one of the following:
yes— Set this interface as the default route.
no— Do not set this interface as the default route.
In the past, I did not use the DEFROUTE option. I found that I could just statically assign the eth0 and *not* let NetworkManager have access to it (NM_CONTROLLED=no). In fact with Centos (and the like), NM seems to get disabled as a generally rule.
Also, if this was a server or I wanted to statically assign the interface, it would not be an issue. Just one of those fringe usage cases.