Don't blame Foster. His actions were merely symptomatic of the difference in approach between the two sides. City were ruthless, we were not.
Look at the drop ball when Milner booted it away from Foster for a goal kick, a much less advantageous position for us than Foster having the ball played to his hands. Look again at the throw in after the Nasri incident. Milner threw it short of Foster, meaning he had to go collect it thus giving Milner plenty of time to get back into position.
Either could be viewed as unsporting, but in reality City are a lethal, well drilled, well organised team with a psychology of never giving up any advantage to the opposition.
Foster's action, in putting the ball out when Nasri was play acting, was just a symptom of Albion's weak, muddled approach. He is an allegedly highly trained professional athlete and should know not to do that, and not to surrender an advantage, unless the opponent is unconscious or clearly badly injured otherwise, or alternatively unless the referee instructs him to put it out.
It was a pathetic moment and just about encapsulated why we are where we are.