It's very much a security thing, there've been lots of exploits that work by dropping a payload into a W&X marked area in browsers (usually dropped in by buffer

It's very much a security thing, there've been lots of exploits that work by dropping a payload into a W&X marked area in browsers (usually dropped in by buffer overflows). Apple also doesn't like unsigned code, and JIT (or self modifying code, or whatever falls into that category) is inherently unsigned code.