Tablet and smartphone manufacturers are doing this because they are trying to differentiate their devices. With the Android OS (along with Android apps) and the hardware converging to a dominant design (ARM-based processors, similar form factor), tablets will become commodities (Armdroid). Competitors will be forced to compete on price. Their margins will collapse.
This is what happened in the PC market. With the Wintel standard, PC manufacturers had little opportunity to differentiate their products. The OS and CPU are identical, as are the internal components and peripherals. What's left? The color of the case? Fingerprint readers? Ultimately, PC manufacturers were all forced to compete on price. As a result, while Intel and Microsoft earned margins in the double digits, PC manufacturers' margins were squeezed to the low single digits.