I totally understand the frustration, and every player does have the ability and the prerogative to 'block' any other player, for any reason whatsoever.
For myself, I guess I'm inclined to be a little more understanding. Presumably when anyone signs onto a game, they are PLANNING to have enough uninterrupted time to be able to finish that game. However, I understand things can happen to change that.
The wife (or 'significant other') suddenly interrupts and must be dealt with. The kids interrupt and must be dealt with. An important phone call or visitor interrupts and must be dealt with. There are any number of things which (realistically) are far more important than playing a game. (And, yes, it is after all still a game.) I do not presume that *I* (or anyone I know only from an online game) would be MORE IMPORTANT than someone's wife, children, or whatever else might necessitate cancelling out of a game.
I'm sure there are others who cancel out for far less pressing concerns. Perhaps they recognize they are just totally mismatched in the game -- either so much worse than the other player(s) that they are embarrassed or feel they are 'holding them back', or so much better than the other player(s) (and so intolerant of poor play) that they are not willing to suffer the lack of ability of others in the game. (Of course, if that is the case, they have the option to pick games that are 'Pro or higher', etc.)
Yes, it is disappointing, even frustrating. But unless someone establishes a PATTERN of regularly and repeatedly dropping out of a game, I don't figure I'd be too quick to judge them without knowing the reason they chose to drop out. I prefer not to be that quick to judge.