my guess is that the bettor on the end forgot about the allin man and thought he was stealing the pot. remember players that are not real experienced do stupid things. i do them all the time as well. if they were really colluding no way would they have the same draw as M pointed out.

as to the unethical nature of a bet into an allin guy, it is totally ethical to bet whenever you want to for what ever reason. ive many times bet into a dry pot just to get out one person and let another win. or to let players see that when i bet into a dry pot i may not have a big hand. otherwise you would never get called in those spots.