Isn't the problem that racists are associating their racism with their Celtic-ness via the Celtic cross, not that ADL is seeing that racists are doing so? I mean, there are legitimate uses of the Celtic cross (e.g., to decorate a bible, as another person pointed out, or to wear as a tattoo if you are an Irish Catholic or what have you), and there are illegitimate uses (e.g., to signify your racist views to other racists). The cross itself does not become racist just because racists use it, yet there are racist uses of the cross. This is really no different than how the KKK burn crosses to intimidate people and such. The cross itself remains the ultimate tool and symbol of Christ's victory over death, but some people see it wrongly and associate it with other things. I highly doubt that the ADL, being a Jewish organization, sincerely cares about its correct use and symbolism, but that's where Christians should be coming in and saying "No! This racist use of the cross will not be allowed to overshadow its true meaning!"
There are still some countries in the world, believe it or not, where the cross still retains only its religious meaning (e.g., Egypt, Lebanon), so you don't find other people taking it in this way. But in Western countries, we have for some stupid reason allowed racists and hip hoppers and all kinds of other people take our most precious sign and use it for whatever reason they feel like. Fashion, racism, whatever. It ought not be this way, and I can think of no better group to condemn than any neo-Nazi types who use the Celtic cross, or any other type of the cross, to signify their hateful ideologies. Even the fact that anyone with a pre-existing religious attachment to the Celtic Cross should have to explain that no, they are not racist by having one, shows that we have allowed racists to associate it with their racism for too long. No one should look at a cross and think "racism", but because racists have used it unabated for decades, this is the situation we are in in the Western world. It's sick, really. Our societies are spiritually sick.