If you fly with a low cost ,or indeed any scheduled carrier, they are perfectly to cancel or combine flights so long as they offer you a seat on the route you booked at some time. A charter carrier has a contractual obligation with the charterer and has to operate the flight. Hence if a charter airline has technical or operational problems they have to find a way around the problem either substituting another aircraft or subchartering.
Charter flights operated by tour operators also have to be bonded so if the airline or tour operator goes bust you either get your money back or if you are on your holidays the bond pays for the hotel and your repatriation.