A corporation is a synthetic legal object: beyond basic restrictions on its form, it's allowed to legislate itself internally according to whatever bylaws and structure it pleases. There is no requirement (and no particular precedent) for them having a democratic structure.
This produces a fundamental tension between the people who comprise the corporation and the decisions that the corporation makes: the corporation can choose to do things that are overwhelmingly unpopular with its employees without significant recourse, since the corporation does not operate according to the will of its members.
In other words: corporations are naturally susceptible to undemocratic power concentrations, where a small number of executives or board members use the financial heft of the corporate body to achieve their personal goals. Allowing those concentrations to then seep into our democratic system is fundamentally corrosive.
> Are you also supporting this position, that government can suppress your speech if any at any point corporate funds are used?
No. I'm not obliged to defend whatever argument the USG's lawyers presented during the particulars of the CU case.