no u

They may not have known they were going to be called in. Technically, employers can call employees in at any time, assuming their availability works with the shift they need covered. Even then, some employers call you on a Sunday when it clearly states you can't work on a Sunday.

Exactly - employers can call anytime they need a shift covered. Therefore, it's a common and logical to inform your manager, especially at a job where they've called you in before, that you absolutely will not be available to come in on that date. As far as them calling you in on certain days of the week you can't work, such as a Sunday, I know plenty of people that say they can't work plenty of days due to religious reasons, but jump on the opportunity to go in on those days if they'll be getting overtime. It doesn't hurt to double check on standing "can't work" days because of people just like this.

For sure. I see your point. I'm not saying it's a bad idea for some employers to call just to make sure you can't cover a shift on a day requested off, I'm just saying that sometimes it happens. When I had a job, I hated how they called me in on days where I had something going already (either my other job or school) but now that I look back, whether I ended up taking the shift or not it just showed that the workplace valued me enough to ask, even though it was likely I wouldn't make it in.