Millions (billions?) trust Facebook too. Yet, Facebook kept the Instagram brand and it paid off.
https://en.wikipedia.org/wiki/List_of_Procter_%26_Gamble_bra...
Presumably they capture more of the overall market for detergent that way. Maybe the same mechanisms do not apply to retail in general.
Therefore making P&G more money.
If you peruse the HN comments from the time, many of them were convinced that this was a waste of money (though not the top comment, interestingly enough): https://news.ycombinator.com/item?id=3817840
It seems plausible that as walmart.com becomes better known as a good alternative to amazon for straightforward items, it might actually make people trust the walmart brand more - and put more shoppers in stores.
Instagram's branding/demo was a big part of what FB was after.