Presumably similar to the accountability that applies to PEs. A smaller group of engineers than all engineers sign off on parts of the system, they're assuming a legal responsibility and the penalties for malpractice can go all the way to imprisonment.

Non-licensed engineer are not liable in the same sense, their employers and PEs are. Same thing could apply in software, but people whine too much and it probably won't happen. Every time the topic comes up, people chime in with "But software is different. It's hard." and expect that lame ass excuse to be sufficiently persuasive. Software is also critical, not all, but a lot. Critical systems warrant additional scrutiny and liability.