I’m from the UK. In most working environments there is an expectation of maturity and responsibility. If you don’t have enough work to do there is an expectation that you, as an employee, are responsible and mature enough to ask your manager for more as ultimately that is what you’re being paid to do - work, whether you like that or not. If you have nothing to do, and deliberately do nothing about that then your employer has reasonable grounds to at least raise this as an issue. If you’re not seen as a someone who takes their job seriously, then you may find yourself looking for a new one if your department needs to downsize, for example.

Also, regardless of whether your manager should’ve known or not, that doesn’t mean your not also at fault for not telling them. If you tell them, and nothing changes, then that’s a different story entirely.

Let me put it this way: if your manager turned around and asked what you’ve been doing for the last X months and your response was ‘nothing’ and then tried to pass that off as their fault, I wouldn’t imagine many employers would be too sympathetic to your arguments.