Remember: If not them, Amazon certainly would have done the same.
"Walmart employs an astounding 2.1 million people. In the United States alone, the company employs 1.4 million people. This is a staggering 1% of the U.S.'s 140 million working population."
Yes there are ways to make sure Amazon doesn't have easy access to it, while still hosting with them, but they are hard, and at the end of the day, physical access trumps most of those security controls.
Also, Walmart are SUPER paranoid about their internal data.
They sell 1/2 a trillion worth of stuff. They have a bigger economic impact on America than google, Facebook,Apple etc.
People really underestimate how much of an impact this has. Here's a completely made up example. Let's say a person of a family of 4 goes grocery shopping once per week. This person buys 30 grocery items on each shopping trip. The difference between Wal-Mart and the local place might only be $0.25 per item. Is it really worth it to go somewhere else to save $0.25? No, unless you're saving $0.25 on everything you buy.
At $0.25 per item with 30 items each week and 52 weeks in a year, this shopper saves $390/year or a little more than $32/month by going to Wal-Mart. $32/month is pretty meaningless to most people who command a developer's salary, but there is a sizable portion of people in the US who could really use an extra $32 each month. That's the difference between Wal-Mart compared to the local shop, and that's the reason why Wal-Mart edged these small stores out of business.
My family actually drives past regular stores because of the selection at a super center.
Maybe amazon would look at the data, maybe they wouldn't but if they did they would do it in a way that was nearly 100% impossible to ever prove.
Corporate espionage is a very deep rabbit hole, and goes on far more often than people think it does.
Why give them the opportunity to do so?