They suffer from a condition called "being human". It causes occasional failures in an otherwise operational controller-human, some very small percentage of the time. Even the highest-quality controller-humans have a non-zero failure rate.
I am certain that every logical person understands and accepts the risk as you explained it.
The fallacy in the argument is that the presence of a camera is going to increase the incidences of failures.
At the core, if being held accountable makes someone perform poorly, they are in the wrong profession and should find another way to earn a living. There are plenty of jobs out there that do not involve a person's actions (or lack of actions) having the potential to detrimentally affect the lives of others.
As an example from the work place, an effective technique for encouraging someone to complete their tasks on time is to setup a regular meeting with them and their peers. Often times I have found that people who seem to always have excuses for why they need just a little bit more time to get something done, all of a sudden, as if by magic, become able to get things done on time if the only alternative is for them to admit to their peers, again, for the umpteenth time, that no... they still aren't done.
While a weekly meeting is not a direct parallel to always on video monitoring, the underlying purpose of providing accountability is the same.
While I have not worked on the rail roads, I do know someone who works for BNSF and he assures me that those engineers have an SOP for everything. Amtrak might not be quite as organized, but they probably have similar training materials. In situations like that, either the engineer is doing things by the book or they are not.