I think it's probably the remnants of the first agreement between the singers and humans. They were allowed to terraform Shinovar, and rule that area, but anywhere else, they were forbidden from. Eventually, it morphed into the "soil lands are for humans, everywhere else is for singers." Then, over the millenia, it became a religious teaching, "don't walk on stones."
Brandon wrote a ton of worldbuilding down before starting to write the first book, and this particular thing is definitely something he planned from the start. He does keep a lot of stuff in his head, but sometimes that shifts over time. Part of our job is to make sure what's in his head now doesn't conflict with what has previously been published.
If the outline doesn't work for something, Brandon will change it while writing. As long as it doesn't conflict with published canon, it's always more awesome than his earlier plans.