It's the app's fault that it doesn't. Apps are supposed to save and restore their state. Mine do.
Unfortunately a lot of apps don't save state, but that's not the problem he's talking about.
It's that once an app sends you to another app, there's no user path context to take you right back to the first app again.
For example, you use a search app to find a restaurant, then you click on one of the results to get menus or reviews. Off you go to Safari, which is fine, but when you want to flip back and go to the next search result... you can't... the search app is gone. You have to find and restart it (and hopefully it's saved state).
It's a real user pain compared to other phones where a simple Back key push pops apps off the stack and you can retrace your path.