This is my view of it as well. If they're not fit to fly, they're not fit to fly.
What I've realized is there might be a disconnect in how people understand the verbiage around this issue. For me, personally, I've always understood fatigue calls to be caused by operational issues at work, whereas I'd be calling in "sick" or saying I need personal time off for things that go haywire in my personal life.
Maybe other pilots are abusing the system, or maybe they don't understand the implications of what they're saying.
That makes sense for some operations for sure. At others, you are not supposed to call in sick unless actually sick so a fatigue call would be the only technically correct way to not work in that case. Also, at some places there is a dependability management program of some sort that tracks a rolling number of sick calls. A fatigue call wouldn’t count against that.