USB-C cables aren't (exclusively) passive! USB-C cables can carry the signalling protocol for, among other things, Thunderbolt 3.1, which is an active protocol (requires powered ICs in the cables), and thus can't simply be opto-isolated by the modems on either side like a purely-passive protocol would be. A given USB-C cable might be a passive component and still be able to wreck the system it's a part of, if the system is active—that is, if the system is constrained from protecting itself against damage by simply re-defining line voltage at each medium change.

This is a compromise most people don't usually realize they're making when they favour the addition of an active "feature" to a previously-passive spec. For example, Ethernet used to be a pure-passive, easily non-ground-referenced wiring standard. But then PoE came along...