I think that VOIPo suggests putting the ATA in front as it eliminates many of the problems that people have with their routers.
Not everyone knows how to properly set up a router, open ports, turn off SPI, etc., and some routers are quite basic and don't really do a good job of routing.
So it's probably a more reliable hook-up for the masses.

Of all the ATAs I have on my shelf, I found the GS to be the most reliable. It just sits there and works around the clock.