Of course non-Whites can be racist as well. In fact just last week some Asian lads were sent down for killing a random White kid in Glasgow.
But it's a little bit like domestic violence; although men can be assaulted by their wives, we all know that because of the difference in physical and economic power, and the way men and women are and are brought up to be, serious violence and terror is overwhelming ly by men against women.
In the case of racism, Whites are still 94% of the population of UK, and an even higher percentage at the richest and most powerful levels of society, including the Police - so minorities are more vulnerable to attack. In fact most Blacks and Asians have experienced racist abuse, and very many racist attacks (this is why so many prefer to live in areas where they feel safe, areas that are more mixed); few Whites have suffered racial attacks, and even when they encounter some form af racial abuse, it rarely carries the same level of threat; at the end of the day, Whites are in such a majority that they have no reason as a group to feel intimidated.
One definition I have seen is that racism = prejudice plus power. If I am prejudiced against blond-haired people, blondies may find me offensive or weird, but they probably won't feel that it makes much difference to their lives.
But if I am prejudiced against Asians, in a climate where many others also are, I am part of a something more dangerous and threatening.
I might add that I am not in favour of different sentences for racially motivated attacks - I mean, motivation has always been a factor in sentencing, but it should have been left to the judge's discretion. We should go back to the priciples of common law, which are perfectly capable of dealing with racist abuse and violence, without giving a false impression of special treatment.