Why is negative id or zero considered a bad practice?

Question

Why is negative id or zero considered a bad practice when inserting a primary key in a database table?

I think it could be useful in some cases, but people say that it is not recommended, despite the fact that they never say/know why.

So, I was wondering if is there, by definition, some restriction or if it shouldn't have any problem or if is it just a convention and if really there is some restriction about that, why isn't that feature blocked?