Design Decisions for using ADC with no VREF
This is becoming a fairly common practice with some manufacturers.  Even some Microchip PICs do this (PIC10F322, If I Recall Correctly).

They do it this way because it's easier for them and not all that much harder for you.  And this technique can work well - IF the Vdd rail is reasonably constant over the time period between calibration cycles.

If you think about the relationship between a measured, accurate reference voltage and the A/D reference varying, you will see that as the A/D reference voltage (Vdd) drops, the apparent reference reading will INCREASE.

That's okay, if Vdd comes from an unregulated battery supply that slowly changes.  It's not so good if you require really accurate readings and your power supply is noisy.

Note that it's also okay if you are doing primarily ratiometric readings such as those coming from some form of bridge-type sensor (strain-gauge, pressure sensor, etc).  

Many times with that sort of product, all you need is low-battery indication.  You don't need to do any run-time calculations - simply read the internal reference and signal battery status depending upon whether the A/D reading is above or below some value that you calculated when writing your code.

For many simple battery-powered products, this technique is just fine.  If you require a better A/D converter, either use a different chip or add an external A/D converter with it's own reference.