Isolation is the reason, not caution
Machinery is safe to work on when it has been isolated, locked out, tagged and verified by someone authorised to do it. That process belongs to the site and to maintenance, and it exists precisely because stored energy, automatic starts and residual movement are not visible from outside. A cleaner has no way to establish that a machine is safe, and no authority to make it so, which puts machine cleaning outside the scope by definition.
The same reasoning covers guarding. A guard is an engineered control, and opening it to reach a surface behind it defeats the control for the duration. Anyone who has been trained on the equipment knows the sequence for doing that safely; a cleaning provider has not been. So the rule is simple and absolute: nothing behind a guard, nothing inside a machine, nothing that requires a panel to be opened.
