^^
What he said ... and for a little history and why it is the way it is now.
Android was always designed to put app data on the "SD Card" when available. When the first Android phones hit the market, they had very little internal memory and a slot for a Micro SD card. You *could* use one without an SD card but you'd run out of available user memory very quickly.
Fast forward a year and you start seeing phones with 4, 8 and even 12 GB of internal memory instead of 512 MB. In order for Android to see the memory it had to be partitioned into system, user data and app data spaces. These are hard fixed partitions. The system took between 768 MB and a gigabyte. User data was 1 to 2 GB and what's left was partitioned to mount at /sdcard. This became the SD card to Android. If a phone had an external SD card slot too, it could not also be mounted on /sdcard.
Therein lies the problem. The path to external storage within Android is /sdcard so apps that are installed to external storage will be there, which is the internal sd partition, not the removable sd card. Since there is no default path for a physical SD card if there is an /sdcard partition, it was up to the manufacturer to set the mount point. (using my devices as examples) On the Captivate the path to the removable sd card is /sdcard/sd_external, on my SGS2 it's /sdcard/external_sd and on my Transformer is /Removable/Microsd. My Nexus 4 has no SD card slot.
Fast forward again to Jelly Bean. With the trend in Android phones having larger internal memory and no SD card slots, Android changed the way it mounted volumes. Instead of static partitions, Android 4.1 dynamically allocates the partitions giving the user access to pretty much all internal memory for apps and data, rendering the ability to "move" apps to the sd card moot. That's why the option is gone.
SD memory is slower and less reliable than internal memory and it's much less desirable to have continuous read/write processes which will both slow performance of your phone and shorten the life of the card.
Let me know at what point you nodded off and I'll repeat from there.
