Well, that is quite another thing. While people are actually under the command and control of the employer, or his representative very much needs to see to the physical needs of his employees. If he doesn't they soon will not be under command and control. Providing adequate opportunity for hydration and bathroom breaks are just basic human needs. You might as well try to keep them from breathing as limits these.
In terms of safety obligations, the employer is required, by OSHA if by no other, to enforce certain prudent safety measures. If you, as an employer, have a prudent policy of not working in traffic without traffic control, for example, and a certain employee consistently works in traffic without traffic control, and you warn him about it but he doesn't stop, and you don't fire is a$$ after a couple of warnings, then you will be liable for the accident when it happens. Your argument that the employee failed to follow the policy will carry no water with OSHA, because you failed to enforce your policy. In OSHA's view your policy was, in fact, to turn your back on safety transgressions.