Because maybe they felt they had enough evidence to do so.

From other people who've posted on here in the past about accounts being closed, it does seem they have a "shoot first, ask questions later" mentality. One could certainly argue that they are a bit shortsighted in this regard as they may be giving up a ton of revenue by taking this action and driving this company full of HVFs away from United, but then it wouldn't be the first decision like this.