Nobody needs booze at all, expat and yet pubs exist.
Is it ever worth making a sensible, factual answer in these threads, because nobody appears to read them? The bars aren't subsidised, in the normal commercial sense of the word. But it's not a normal place, not least because it's impossible to gauge the cost of, for example, premises and security, or separate them out from the costs of the Palace of Westminster as a whole. The costs of running all the catering outlets together is more than the cost of the food and drink paid for, because this is not a normal commercial outlet, and operates even where there is little turnover, and in circumstances where the clientele is incredibly unpredictable (recesses and snap elections, anyone?). Plus the catering staff get exciting things like pensions.
Here's the response to a Freedom of Information request about the costings.
It is important to note that catering services for the House of Commons are provided by an in-house team who do not provide a subsidised service in the commercial sense of the word. However, the overall cost of providing the service does exceed the income received in sales due to the irregular hours and unpredictability of parliamentary business. The House monitors the contribution or cost of each venue. The contribution or cost of each venue is calculated by subtracting the food and operational costs from the catering sales. Prices of food and drink are regularly benchmarked against appropriate external comparators. Please also note that the customers who use the on-site catering venues include some of the 650 elected Members of Parliament. However, they also include around 14,500 other pass-holders (MPs’ staff, House staff, civil servants, contractors’ staff, Peers, members of the Press Gallery, etc.) as well as a large number of non-pass holding visitors to Parliament.