PerlMonks strives to minimize the impact of "editing" of original posts. Site Janitors do their best to walk lightly. Whether it be by human touch or automation, changing the code's layout ala perltidy would break away from the spirit of that philosophy. Speaking for myself as a janitor, I have evolved to prefer simply wrapping naked code in code tags without altering the code's actual layout, as much as possible (or more accurately, as little as possible). I wouldn't want to automate something more heavy handed than that.
As for syntax highlighting, I know that though there are many proponents, there are some who feel that it encourages sloppy code layout.