ADS8671 ADC: 1 MSPS, but 15 kHz antialising filter?
I believe this answer on the DSP StackExchange is why. As Andy mentions, 20dB for an AA filter isn't all that great, if the Nyquist frequency isn't much higher than twice your sampling frequency. But, by pushing your Nyquist frequency far higher than your AA filter's cutoff, the magnitude of any aliased signals will be reduced markedly, while reducing the cost of the chip as a whole. 

I've wondered this too and never thought to ask about it, so I (pending criticism of this response) learned something here :)

As far as your question about "typically set", I will hazard a guess (though I can't ascertain what the authors meant): the frequency response of the filter appears to change somewhat with different Vref's, and as such the -3dB frequency shifts slightly, "typically" at 15kHz.