In the 2000 Summer Olympics in Sydney, Eric Moussambani, from Equatorial Guinea, was invited to the games under an IOC program that allows smaller countries to compete without meeting qualifying standards. He swam his 100m Freestyle heat in a heroic 1:52.72, and won, due to false starts from his competitors. Why didn't he go on to compete in the semi-finals? Did he opt out, or was this program essentially for exhibition only?
As you can see from the results, there was no "winner of every heat qualifies" rule; it was simply the fastest 16 swimmers which went through to the semi-finals - note how all the semi-final qualifiers came from heats 7, 8, 9 and 10 (which were loaded to have the fastest swimmers).
You could view this as being a exhibition heat, as there was no realistic chance that anyone from it was going to get into the semi-finals - however, you could also argue that it was better for the sport to have Moussambani competing against swimmers closer to his skill level, rather than tailing in very, very last against some of the best swimmers in the world.