VAT is a tax on consumption, corporate taxes are on profits (revenues in this case). If Amazon does not have to pay as high corporate taxes due to transfer pricing, then it can offer a lower net (pre-VAT) price so their products end up being cheaper to consumers driving away sales from other retailers. While competition, you may say it is 'unfair' competition.

And yes, this makes it very hard for new businesses to enter the market as they do not have the resources necessary to do tax optimization.