That's your browser doing that. If you changed your items to Walmart Only, please take a look at the supplier url. As long as the supplier url has #skuopt:walmart at the end of it, then all is well. Sometimes your browser doesn't allow the drop down to stay changed from Default to Walmart Only, but please note that the only purpose of the drop down is that when it is selected, it automatically applies the #skuopt:walmart tag to the end of your supplier url. As long as that is there, you're good. There is nothing to worry about.