Although I am not a fan of Apple, their Memory Pressure messages on iOS are really useful to prevent this. You get warned as a programmer to clean up your memory if possible, otherwise iOS shuts you down.
What are you going to anyway, just start deallocating the memory your application needs to actually work, so that you can display an "I'm sorry, we're out of memory" message? Might as well crash then.
The amount of memory available also varies wildly from user to user and you can't put a memory requirement on an application.
So what do people do? Disregard the issue. Even the best games on the App Store have tons of one star reviews by people with older devices.
The flip side is that application that are prone to crash are pressured to optimize for relatively quick and painless restart.