Grout is not waterproof. Some water will get through it. My
understanding is that backerboard absorbs some water, which then
evaporates into the wall space and through the other side. I was
told NOT to use a vapor barrier on the back of the backerboard to
avoid mildew problems. It traps the moisture in the backerboard. I
assume the same would be true for floors.

So what about tiled walls in the shower.. the water impacts directly
on the tile and grout...

Grout is water-resistant, but not waterproof. It can stand up to 15
minutes of water, especially on a wall, but not to hours of exposure.

Shower floors have a waterproof pan underneath to stop the water that
gets through the grout. Walls rely on you getting out of the shower and
letting them dry.

It's _still_ a good idea to seal the grout in the shower.

--
Steve Bell
New Life Home Improvement
Arlington, TX USA