I think it depends on the airline. Most of the budget European airlines I fly with will allocate you a middle-seat by default if one is available when you choos

I think it depends on the airline. Most of the budget European airlines I fly with will allocate you a middle-seat by default if one is available when you choose the free "give me a random seat" option at check-in. I presume this is to give away their paid seat inventory as slowly as possible, and also because by giving the passenger the worst available seat you get more value from paying to choose your own seat. Of course: they have a limited number of middle seats to give out so I've found that a good strategy is to check-in five minutes before the online check-in cutoff time which usually yields window/aisle seats and frequently (~30%) exit-row seats.