"Predatory pricing to push companies out of business is absolutely illegal"
It's impossible to prove in most cases because it may have nothing to do with intent.
Amazon's advantage in 'groceries' is inherently structural.
They can sell bananas for 10 cents less than Safeway because their opex may be subsidized, or they can transport cheaper, not 'because they are acting predatorily'.
The nature of this value chain overlap is inherently problematic.
The thought of 'putting grocery chains out of business using surpluses from AWS' never has to even cross Bezo's mind, or the minds of anyone in the entirety of Amazon. It can happen due simply to structural issues.
For much the same way, it's impossible to have a free trade deal with China, so long as they have political controlled financial system and currency. 'Dumping' will be an artifact of the system as a whole. Nobody even needs to think it for it to happen.