The government can't do whatever it wants with state aid but the money that goes from the government to the airport is a loan on commercial terms and therefore isn't state aid. The airport will negotiate with a new entrant depending on how many routes and passengers they are offering. The long term risk that they will use those terms to drive away the competition and in the long term leave you with an airport dominated by one carrier is well known by smaller airports that have negotiated with the LCCs.