Amazon knows most of that $15/hour will be paid into Amazon - regardless of employer - as they have 51% of American commerce. They're turning America into a huge company town.
* Total US Retail sales in 2018: ~$5.4 Trillion
* Amazon trailing 12 months revenue as of June 30, 2018: $208B.
This is a little less than 4% (and that's assuming all Amazon revenue is US-based and retail sales, rather than AWS or other...)
You may be thinking of share of growth of US eCommerce.