Unfertilised eggs come from chickens. Vegans I've spoken to tend to object to farming.
Even though I wouldn't eat their eggs, I can't really see the problem with having chicken around if they don't suffer. For people who eat vegan for ethical reasons, my impression is that suffering is the main concern.
That sort of logic gets you back to farming animals for meat though. We're giving them a relatively cushy life compared to wild animals (at least in some cases), but cutting it short. It's easier to maintain the moral high ground if you simply cut out all animal products period.
My point was that I find it surprising people with this position who "break" for medical or energy reasons don't generally arrive at the shellfish conclusion. They eat eggs, dairy, meat or fish.