It's a common behavior in SPA that's uncommon elsewhere. Kind of like how buffer overflows aren't C behavior.

While I agree that the back button _should_ work in an SPA, it's common for it not to. And it's common for other functionality to break. It's not hard to fix that and, for a well engineered SPA, such things typically do work. However, it _is_ extra work and it _does_ get missed. As such, SPAs tend to have more issues than non-SPAs. As such, as a user, I tend to prefer non-SPA implementations unless there is a strong reason that the SPA makes more sense.

It’s very common in poorly engineered SPAs. “Built in” browser functionality often has to be reimplemented.

Not only is it not implicitly SPA behavior, as you say, good back button behavior design is actually hard in a app setting. And you can’t generally “get it right” — no matter what you do, in a sufficiently complex scenario (which… that’s why you built an SPA, right?) users will disagree about what the back button should do!

Generalized anti-SPA sentiment is silly. Saying “bad implementations correspond with SPA” is true but also silly because there’s not as much to get wrong in non-app settings which admit a “traditional” approach. It’s like the xkcd observation about heat maps and population density.