Short answer: because there's a difference between apple pies and pies with apples.
Longer answer: because the sentence that's being translated is talking about apple pies and not pies with apples. Basically I don't actually know, but I know in English you say Apple Pies rather than Pies with Apples.
Not sure really. Anyone else know?
Shouldn't the translation be "Eu am doua placinte de mere"? Similar to the concept of something like "Eu beau suc de portocala"? What if you arrived to a friend's place and wanted to say "I have two pies with wine"? "Eu am doua placinte cu vin"? Obviously you wouldn't mean you have two wine pies...