I'm not seeing that exact CSS rule in Flarum. Are you referring to inline code (``) or code blocks? ( tripe backtics / [code][/code]). I'm seeing an overflow: auto rule on pre elements however.
If you're talking about code blocks, I suppose it's so the code doesn't overflow the post boy to the right, hiding the right navigation and breaking the forum UI.
EDIT: nevermind, I completey missread your report. I thought you had seen the overflow-y.
I suppose it's just neither a native browser feature, not a feature of TextFormatter and neither a feature of the hljs plugin used to highlight the code, so there was no reason to do extra work to implement that. This could be added by an extension relatively easily.
