The difference comes down to market share. It's fine to sell store brands if you have 10% of the market because there's plenty of competition. Vendors who refuse to deal with you can do just fine by working with your competitors.
Amazon is approaching 50% market share. They are close to becoming the place for online commerce. So when they compete with vendors by offering similar products, the vendors are effectively put out of business.
Just like with MS and IE. The issue wasn't so much that MS offered IE with Windows so much as the fact that 95% of computers came with Windows, so bundling IE was putting other software providers out of business.
