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.
Islam also has a ban on usury, which has led to some novel banking practices to provide the effect of modern banking while retaining the ban. For example, in an Islamic "mortgage" what happens is that rather than a loan, the "bank" retains ownership of a declining portion of the property, so instead of paying principal and interest each month, the purchaser increases his ownership stake (principal equivalent) and pays rent on the portion he doesn't yet own (interest equivalent).
