Charging interests does not equal usury. Usury is exploiting someone else's desperation/ignorance or weak position in general to charge excessive interests. There will always be a subjective element in the definition of usury.
That's how it's defined now, but I suspect that it's defined that way partly as a rationalization for what a modern economy requires. In the time periods and places in which usury was something people were really up in arms about on religious grounds, what constituted it was a much lower bar.
