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?
Feminine and neutral nouns can also have -e endings (along with -i, -uri, -ele... ). Masculine nouns are the only ones that end just in -i, if I understand it correctly.
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...