And no, it’s not acceptable to expect them to simply resign. Changing jobs can also incur significant costs on people, which can allow employers to exploit their position of power over employees.
When an employment contract talks about working hours, it should not be necessary to refer to small print or seemingly unrelated clauses to know what that means. It should have a straightforward and unsurprising meaning. Employers should not be able to make up surprising and counterintuitive definitions of things like working hours, and manipulate those in their favour. In general employees should not be required to understand special exceptions from standard expected practice, unless there are specific well understood and communicated reasons that require it.
On the face of it, I don't think this case meets that sort of standard. In general I am in favour of a high degree of flexibility in the form of employment contracts. My own working hours and the expectations of my job are somewhat unusual. I work highly variable hours and I am frequently on call. I do get paid for the hours I work though.
However I do think this is a reasonable area for legislation because I think that's the only way to ensure employees are not abused, and in this case I think it's pretty clear Amazon did not signal this clearly to prospective employees and is abusing it's position of power over them in an unreasonable way.