The original law says that beer should only be made from barley , hops and water. Note that this is not barley malt, but barley, which is specified. Of course, no mention of any other form form of malt or grain, such as wheat. Guiness, which doesn't count as a Reinheitsgebot beer because of the use of roasted barley would have qualified under the 1516 rules. German brewers do not always stick to the Reinheitsgebot themselves .
Many breweries use various adjuncts for versions of their beers sold abroad.
Many German wheat beers may not, strictly speaking, be sticking to the rules of the Reinheitsgebot . As this yeast will be continuing the fermentation in the bottle, it's a matter of debate whether the end result is a pure top-fermenting beer. Belgian fruit and spiced beers, Finnish sahti, even traditional Guinness, would no longer be possible. Then consumers can see what they are getting and make an informed choice. Personally, I don't want to have my choice of beers determined by medieval legislation to stop Bavarian peasants malting grain they should have been turning into flour.