I did the piss test prior to my current employment and haven't in nearly twenty years since then. About the only chance it will happen is if you give clear signals your messed up.
Plus many industries are required by law to test. Considering the breadth of WalMart operations its likely they cross one of those laws and to avoid discrimination they test everyone.
[1]: https://hiringcenter.walmartstores.com/OnlineHiringCenter/di...
It's not about drugs (though that really shouldn't be a concern). It's about respect, privacy, and power relations.
There are companies which have mandated drug-testing policies by regulation (mostly involving transportation or pharmaceuticals). It's still a major chilling factor so far as I'm concerned.
I wouldn't drug test employees, but I can understand why some companies might be compelled to do so. I have friends who smoke pot recreationally, and they're fine. I have also had friends/coworkers who became genuinely addicted to drugs, and very consistently they ended up as ticking time bombs for their employers. Things didn't end poorly, they ended disastrously. I know that's not always the case, and that it doesn't justify drug screening everyone, and that it's an unpopular opinion here. But I've seen enough that I know why some employers think it's worth it.