Is their argument that it harms customers and competition? It seems it benefits both. Customers get cheaper goods and Amazon is competing with the existing play

Is their argument that it harms customers and competition? It seems it benefits both. Customers get cheaper goods and Amazon is competing with the existing players. The government willingnes to act is then easier to explain: it's not defending customers (it rarely does) but it tries to shelve other established businesses from competition (which is common).