I'm not sure what you mean by "it applies at every company". Getting hired and then fired a week later is virtually unheard of. This is not a fear I have, at all.
But if you told me it's probationary, that is totally a fear I'd have, I'd get paranoid, so I'd rather work somewhere else. You're basically telling me it's not a real offer in my eyes, and I should not expect stability.
> Again, in practice it seems mostly harmless.
It's extremely harmful in a place with poor labor protections that is the US, for reasons that I don't feel like expanding on and that you can educate yourself on if you wish.
Not everyone likes it or agrees with it, and I can only comment on the software industry here and not other industries but it's not the end of the world and the sky doesn't at all fall. When they introduced it a lot of people tried to make arguments like it would be abused etc and as far as I can tell there hasn't really been any drama. YMMV depending on country.