OK, so the 'conspiracy theories' are that they removed the support to discourage piracy (or at least make it more difficult), that they did it to boost Cloud storage (but since they don't let you install apps in the cloud or use them in the cloud, that's doubtful to me), or that they did it because they want to be more like Apple.
The official statement (which, does make a lot of sense if you take it with a pinch of sugar) is that they did it to improve the overall efficiency and to reduce complexity to the user. Sure, most of us are tech-savvy to a certain degree and we can figure out a few options in the settings, but there are users that aren't and that just want their phones to do what they're supposed to do and no questions asked (this is where iDumb devices excel). Also, reading from an external source meant using resources to mount said storage and use that as a system partition as well, so the OS efficiency bit is actually true; you can remember that widgets couldn't run if they were installed on the SD card, right? Well, now imagine your card fails (like the SanDisk cards tend to do in newer Samsung Devices) in the middle of something or that some app was stored in the SD card and you remove it... The system goes wonkers because you probably didn't stop the app from doing what it was doing and now some random instruction or operation is lying around your system making your phone unstable or using resources on a pointless task.
The one thing I do miss though, is that DEVELOPERS have the option of installing secondary data (i.e. games) on the SD card, but most of them are too lazy to implement it, so they default to internal memory instead.