There is one slight difference in the reasons to update or not update apps: Incentive.
Samsung has no motivation to remove the menu key because it actually makes their competition look worse. If they lapse and have a black bar menu soft-key appear, Samsung can point and laugh and pretend it's the other OEM's fault, and not Samsung. Developers, following Samsung's trend (due to their vast majority market share) don't have a lot of incentive to adopt unless they're hearing from customers that they're not using the app because it's not updated. It's the developer's fault, but the customers blame the OEM. This is compounded by many more apps being free on Android than on iOS.
Samsung is not likely to change soon because they have the same configuration (less one search key) in many generations of Galaxy devices now and making that change suddenly would be off-putting to many repeat customers who now don't know how to find menus. That's a big part of why they haven't even moved the back key to the 'standard' place that it sits on pretty much every other device.
Apple developers are all working with the exact same set of customers, many of whom are on the one (now two) latest device that has a scaling issue. They also are fighting for dollars, and ranking in results searches meaning that they want to keep as many customers happy as possible and there is no other OEM to blame. If an app sucks, it's the developer... people are usually not blaming the phone maker in that case.
This, in my opinion, is a prime indicator of why in roughly 12 months, many if not most apps, have been corrected to work on the iPhone 5 (and now 5s and 5c) sizes, while there are many and more apps on Android that ignore the guidelines regarding menus and settings.