Without Root, why can I pull a specific apk using adb, but not everything in /data/app. Android 4+?

Using: Droid Bionic 4.04 without Root. I can use the adb to pull a specific apk file to my PC, for example, adb pull /data/app/com.google.android.gm-1.apk C:\my-hacks which pull gmail's apk file. But when I try to copy the entire /data/app directory with adb pull /data/app C:\myhacks the command line reads
pull: building file list... 0 files pulled. 0 files skipped. I don't understand **why** I have permission to pull specific files from /data/ but not everything.