The world can be a dangerous place. You wouldn’t think that getting close to a fence would be risky, but then, a fly doesn’t think a venus fly trap looks risky either.
Being a fence is a pretty passive existence. You don’t travel and pretty much just hang out in one place until you rot, rust or get removed. Some fences use this time to improve themselves in a good way.
Others evolve less beneficial features. So it’s important not to take a fence at face value. It may have evolved and have an appetite for arms.