When an app first requests access to photos, it’s one of the options listed in the system permissions dialog, so it’s virtually the default. The problem isn’t that, it’s that once you’ve picked the “selected photos only”, apps can choose to make it a pain to pick additional photos if they don’t add a UI element for it. Given that Instagram had it before and then removed it, I can only assume that the real reason is to try to coerce users into granting all access (nice try FB, but not going to happen for me!).