Even if they're taking huge risks going through a debris field, they're destroying the craft soon after anyway. If it actually is risky, it's probably worth the risk.
They might not have known the gap was there, either. Much of what is now known about the structure of Saturn's rings was first discovered by the Voyagers.
Perhaps I'm putting too much emphasis on the naturally pristine state of things around Saturn, but I've read that also is a priority of researchers, at least in some circumstances.