That's shoving a square peg into a round hole. Amazon's competitors have the same labor pool for their warehouses, so it can hardly explain an 80% difference in injuries.
The solution is simple then: Amazon should allow Walmart employees to vote on Union issues...
If Amazon and Walmart warehouse workers want to form a joint union there is absolutely nothing stopping them and it'd probably be a pretty good move for worker's rights. But, while Amazon is clearly anti-union, Walmart hates them vehemently.