The only thing that really is trust abuse is lying, with a few exceptions.
That is a ridiculous assertion. Both positive and negative ratings have a clear description and if the situation doesnt match the description (reasonably), it is abuse.
Being a troll has nothing to do with the chances of someone not honoring their obligations in a trade, which is what the trust system is supposed to help people measure.
The same can be said in many other reasons for giving out ratings.