Lawrence can explain it to you:
"People who invest in business corporations, as opposed to contributors to ideological non-profits of the sort that Citizens United itself represented, don't typically intend thereby to authorize the managers and directors of those corporations to use the money invested in their businesses to help some candidates win election to federal office or to hinder the efforts of others vying for positions of federal authority. Talking about a business corporation as merely another way that individuals might choose to organize their association with one another to pursue their common expressive aims is worse than unrealistic; it obscures the very real injustice and distortion entailed in the phenomenon of some people using other people's money to support candidates they have made no decision to support, or to oppose candidates they have made no decision to oppose."