Like some people said already, its mostly the stigma, and I_P terrorists (Richard, WN etc), who were infamous for using alts.
I think it also plays a part in wether someone can trust another, as now, the almost constant finger pointing, we can't really trust others anymore, for fear of them being a terrorist alt.
Also, that's why this community comes off as snobbish, or closed to new users, because, after all these terrorist attacks, we're like a healing wound. We are better off than before (because we dealt with the threat), but we need the chance to heal.
Alting itself isn't a problem, its just how alting is used and perceived.
TL:DR: Alts are not the problem. How we see and how ppl use them is