Why doesn't the burning charge sneak around the apex seal at the spark plug and ignite the intake charge prematurely?

 

The short answer is that the end gases are difficult to ignite and haven't started burning yet when the rotor gets to that point.

The slightly longer answer points out that this is exactly why the trailing ignition feeds through a tiny hole rather than a full diameter hole like the leading. When the apex seal swings past the trailing, there is a high pressure difference between the relatively recently ignited mixture, and the just started to compress mixture in the following rotor face. Meanwhile, when the apex seal goes over the leading plug hole, the mixture burning is about as complete as it will get and the chamber is mostly expanded anyway, while the following chamber is most of the way through the compression stroke, so the pressure differential between the two faces is very low (and depending on factors, might actually be going the other way).

Another question to ask might be, why don't bridge and peripheral ports that suffer from hangfires/misfires backfire through the intake?