It is just not submitted to the Namecoind in the first place, so there's no test as to whether or not it's a valid share. It's kind of a pointless feature IMHO, but I had a couple people ask for it. Maybe some people just hate Namecoin?
Good. I would imagine that some people opting out of merged mining are specifically wanting to not support the Namecoin network. Honestly though, knowing the mining community I would expect most people opting out mistakenly believe that doing so will boost their Bitcoin income.