Feature, not bug. I don't want players to use old versions of my games (so that e.g. I won't continue getting bug reports about stuff fixed long ago) & by doing a web game you get free auto-updating.
Yeah. With iPhone development (where users can indeed hold onto old versions of an app if they want), we often force people to update remotely (lock the game if they have an old version) because every time we release content bundles and the like we need to make a different version of them for every single live version of the game. That means doubling, tripling, or even quadrupling the amount of work in some cases. No fun.