"But does this really make an overreaction legitimate - That other countries are doing it too?"
No, but it can help calibrate whether people really are overreacting. I find it is quite common for people to decry their home country for doing something that they seem to think is egregiously bad when it is in fact something done by lots of countries and something that has been done for lots of history. This may lead you to condemn those other countries and the "lots of history", too, but it does tend to at least tamp down on the idea that we're doing something uniquely new and horrible which could lead to uniquely new and horrible outcomes.
I think there's some truth to the idea that what HN is complaining about is not effectively securing our borders, and that this is at best a waste of time and at worst a process that will generate a lot of false positives. But there is also some truth to the idea that some people seem to have picked up the impression that the standard around the world and throughout history is that border agents just wave people through every border without questions and with a smile, and that therefore any scrutiny beyond that is some sort of unprecedented betrayal of the very foundation of civilization.
Or, if you prefer, the US may be overreacting at the border, but it's probably overreacting a lot less than people think relative to what is common in the world and throughout history.
Then there's a relatively new issue (on a civilization scale, anyhow) of what to do about cell phones and computers at the border. Again, I think, once you get rid of the idea that borders examining what kinds of things you're bringing in with you being some sort of brand new imposition unprecedented in the history of the world or Western civilization, you can get a clearer picture of why this is such a confusing problem. If it isn't that abnormal to look at people's papers they're bringing in, how do you deal with the fact that people can nowadays literally slip the entire contents of a library down their sock? And, on the flip side... why exactly do you care when they could also just have SCP'ed that entire library over too? When this topic comes up, we often discuss how easy it would be to get around this by simply carrying in wiped devices and recopying everything we need over the Internet afterwards, which is a challenge to the entire reason the border agents think they have an interest in this matter.
My point here is not to defend or attack our current practices. My point is that if you consider these issues from a historically-informed perspective about how borders have historically operated in the real world, as opposed to how you think they ought to operate combined with the ever-present human cognitive temptation to conflate the is vs. the ought, that the current confusion around how our borders function in a highly-connected world at least starts to make more sense. And if you want to have an effect on how we go forward with our border policies, it is always a good idea to have a better understanding of how we got here rather than a worse one. I would imagine some people are trying to play the "but other places have it worse so who cares" game, but I think some people are also trying to do a bit of education as to how borders really work.