BEWARE! When it comes to SD card support, there's no such thing as an "Android phone." Each model has its own custom OS with its own custom rules. And, just because your phone has a slot for an SD card doesn't mean the SD card is supported in any meaningful way. Many devices with SD slots deliberately cripple SD card support, preventing you from moving applications there or making the SD card the default for data storage. It's unlikely that you will find out that your phone's SD card support is sodomized until you have already purchased an SD card, installed it ad tried to use it.
Try rooting the device and see if that opens up the possibility of using the card for storage. Many Apps refuse to be moved to the card or to work properly because they depend on living in the syystem/apps folder. However, an SdCard can be a good backup storage place.
Copy all the photos, videos, music etc. and paste them on to the card then when the device is turned off, remove it, slave to it your PC and back it all up. Then clear the card and start again.
They are Android, just altered version of it. They aren't other OSs.
Additionally, Saga Lout is right. You can always take the risk and try rooting the device.
Just like any device (computer, etc.) the company who makes it has the right to make it work how they want. Just like we have the right to not purchase it.
There are two sides to it being blocked by some companies. Yes, it could be due to their wanting someone to purchase a device with more room, or other options, but that isn't always the case. And from the other side, when someone does convert a SD card from "External" to "Internal" storage there is a risk that people usually don't think of. The card becomes encoded and only works with that device and in that devices present condition. If you try to use the card in another device it won't be able to read it. Additionally, if you reset the device, be it a full reset or the kind that allows you to keep your data (which is not really a reset, but that is a whole other subject) then you lose all access to the data on the SD card.
It isn't fraud. There is no mention in the details when purchasing a device that says you can put all apps on a card.
Now if you wanted to go down the possibly fraudulent line try looking at the storage space (for almost anything). That can be seen as possibly fraudulent in a couple of ways. 1. Because the advertised amount is not the amount you have access to, and no where do they mention how much space you actually have access to (this refers to Android devices), and 2. The way they calculate (and this is true on many things (it is a pet peeve of mine) the space is incorrect. It is done using 1000 bytes rather than the correct 1024 bytes. That can add up to a lot of missing space.