In the physical space, Walmart is still king though, so the same concerns that apply to Amazon's ecommerce would also apply to Walmart's in store product placement.

The anti-trust concerns would also apply to Walmart's own ecommerce. They're effectively leveraging their brick and mortar presence to establish their ecommerce business (which is presently the 2nd largest in the US). The 3rd largest is eBay, which leaves one to wonder how much of the hole left by Amazon would be absorbed by Walmart.