The big difference between Gatwick and Fiumicino is simply... the Brexit! When getting into the non-Schengen area of any airport in the Schengen zone, one "legally" leaves the country. To get back to the Schengen part of the airport one has to go through immigration. So, while getting in and out of a country is not forbidden, the extra "work" required simply makes airports say one can't do it. In the UK they have no problems of the kind, I guess it's simply an organizational decision that forces you and others to either pay for your drink/food at one of the restaurants in "your" terminal or a way of saying "go to the @#*&$ing lounge you are supposed to go or just stay dry!"