Boss adds new job responsibilities that weren't in the job description every week, tells employee she should be grateful when she pushes back: 'I’m watching coworkers burn out left and right from the same creeping scope expansion.'

They're too cheap to hire another employee, so they have one employee doing the work of three.

It's normal for a job to increase in responsibilities over time, especially if the new responsibilities align with the primary ones. A manager or boss might want to see whether their employee can take on more responsibility; if they swim rather than sink, they might be eligible for promotion. 

There's nothing wrong with a gradual increase in responsibility over time, especially if that's what the employee is asking for. But if every week your boss asks you to do a different task without even giving you a chance to master the last new responsibility you took on, they're not setting you up for success. They're betting on the idea that you're a hard worker who is hard-wired to do everything you're asked to do, no matter how much time it takes or how much it stresses you out. And if you're a low-level employee who's just trying to put food on the table, you're not really in the position to turn down responsibilities at your job, even if they were never included in the job description.