"Shamroguery", that would be a brilliant name for a commercial beer.
Putting my oar in here. While water chemistry is mostly about getting the pH right in the mash, the salts in your brewing water do have an effect on the flavour of the beer, quite a part from pH. For instance, you could use Calcium Chloride or Calcium Sulphate, more or less interchangeably to affect the pH of your mash, but the flavours in the resulting beer would be different depending on which you used. Calcium Chloride tends to accentuate malt flavours and mouth-feel, while Calcium Sulphate accentuates bitterness and hop flavours.
This is why it would make sense for them to top up with RO water. If the local water had a bunch of some salt or other in it, it could affect the beer in an unpredictable way.
The same could be said for extract homebrew topup water. If you have worked out salt additions in the main boil to give you a particular flavour profile, adding water with a bunch of unknown minerals in it might skew it. I can't see it being much of an issue though. Most extract brewers make great beers with whatever water comes out of the tap. You only need to worry about it if you have consistent flavour problems with your beers. If hop flavours are dull, you might need to add a bit of Gypsum (calcium sulphate) for example.
