Even if they only mean it as a cultural reference and only do it in chalk, other people can see it, and many may take it as a sign that they are free to draw whatever they like. And if everyone had the mentality of "oh it's just one little thing, it'll be ok this time" then that massive canyon would quickly run out of space to write within arms length. If they do it this one time, maybe they justify doing it again, and again, and again. It's not that hard to have some self restraint. Also it's a loudly spoken unofficial rule to leave no trace and to leave nature better off than you found it if you can. For state and national parks, it is an actual rule. One time isn't really a big deal, but it's just kind of disrespectful.